AP Biology Development

Welcome to Educator.com.0000

In this section on animal development, we will be picking up for the lecture on reproduction left off.0002

That lecture finished out with fertilization, the union of sperm and egg to form a diploid zygote.0008

And you are going to notice in this lecture more so than the other physiology lectures that we will be talking about different animals,0015

some of which are not even mammals because a lot of the work done to study development has been done on various animal models.0023

Following fertilization, the zygote, which is a single cell,0033

undergoes mitosis and very rapid cell division one after the other with very little growth between cell divisions.0038

The result is that the cell stays or the embryo stays about the same size overall, but it is subdivided into smaller cells; and these cells are called blastomeres.0049

So, during cleavage, there is a rapid series of cell divisions with little growth in between.0062

As you can see, the embryo stays the same overall size, but it is divided to two cells, then, the 4-cell stage.0083

So, we have the 2-cell stage, the 4-cell stage and the 8-cell stage. Each of these cells is called a blastomere.0091

Eventually, these cell divisions are going to result in a solid ball of cells, and this is called a morula.0104

Eventually, a morula will form, and morula is Latin for mulberry because of the appearance of the morula.0115

Now, there are a couple types of cleavage, so there are two ways that cleavage is categorized.0127

One is called meroblastic cleavage, and this occurs in animals such as birds and reptiles that have eggs with a lot of yolk.0134

And because of the yolk, the cleavage plane actually cannot cut through and cross when there is a lot of yolk.0158

And the yolk contains nutrients for the growing embryo.0167

So, in meroblastic cleavage, cytokinesis cannot be completed where the yolk is.0170

Cytokinesis cannot occur where the yolk is, so the result is uneven cleavage.0177

In other words, cleavage will only occur...if the yolk was down here, then, cleavage would just occur up here in this half.0193

What you are seeing here is the other kind of cleavage, and that is holoblastic cleavage where cleavage occurs evenly.0203

The cleavage plane can pass through all of the cells, so if there was a bunch of yolk over here, this cleavage plane would not be able to be completed.0211

And then, cell division would just be occurring on this other half, and that would be meroblastic cleavage.0220

In holoblastic cleavage, then, the cleavage plane can pass through anywhere in the embryo, and this occurs in the eggs.0226

So, holoblastic cleavage occurs in the eggs of animals that have just little amount of yolk. Mammals undergo holoblastic cleavage.0239

I want to review some terms that I introduced in an earlier lecture when we talked about classifying animals.0259

Animals, one way to classify them, we talked about body symmetry and various ways of classification.0266

And one is according to patterns of embryological development.0271

Now, recall that protostomes have spiral determinant cleavage.0275

Recall that determinant means that even very early in development, even as early as the 4-cell stage,0290

there has already been some fate established, that a particular cell cannot just become the whole organism.0297

It has already got a certain general fate, so there has been some determination of what this region, for example, would turn into.0305

Deuterostomes exhibit a pattern of cleavage known as radial, and this is indeterminant cleavage; so the cell fate is not already totally set.0316

And if you took one cell from an early embryo, let's say at 2-cell stage,0334

it could even develop into a whole other organism, which is how identical twins occur.0338

Vertebrates are deuterostomes, and the other term that you should be familiar with is totipotent.0347

When we are talking about determinant and indeterminant, totipotent means that a totipotent cell is one that can develop into any cell type.0359

Its fate has not yet been determined.0376

Now, the farther along in development an embryo is, the more restricted the fate of a cell becomes.0380

So, with each division, with each change, the chances of a cell being totipotent are smaller.0387

Alright, following cleavage and morula formation, the next step is the formation of what is called a blastula.0398

What happens is a fluid-filled cavity develops within the morula.0407

Now, this just shows the 8-cell stage, so there is a lot more steps in between.0412

But just to give you the idea, going from a solid ball of cells to a blastula, which is going to have this fluid-filled cavity called a blastocele.0416

And the way this results in general is that as the cells divide, they, sort of, push up against each other on one side.0429

And then, this can flatten out even more, and cell division continues; and what you end up with is a hollow ball of cells.0437

So, the blastula is a hollow ball of cells with a fluid-filled cavity.0444

This is the general idea, and I am going to talk about mammalian blastula formation, bird blastula formation.0460

And there are different terms for those type of blastulas, so this is very generalized; but the specifics of each species are a little different.0469

And sometimes, the terminology is different, as well, but in general, blastula formation is something along these lines.0477

OK, so, we are going to start out with the generalities and then, look at some specific types of organisms.0486

The next step after blastula formation is gastrula formation.0493

During this stage, cells also begin to differentiate, and this gastrula formation occurs as some of the cells...so, here we have the blastula.0499

And what happens is some cells from the blastula will actually migrate in here.0511

And there is also going to be this invagination or infolding along one side of the blastula.0517

Now, introducing the term morphogenesis, which is very important, and morphogenesis means the beginning of shape.0528

During morphogenesis, the shape and the organization of cells are changed.0542

As morphogenesis continues, organ structures, tissues, all differentiate, and cells become more and more specialized and grouped.0547

So, during gastrula formation, morphogenesis is just beginning, and this reorganization of cells, you cannot see the way this is cross-sectioned.0556

But, there is actually - it is a little harder to see - three layers. There are three layers of cells with the gastrula.0566

And recall, when we talked earlier on, those three layers are the ectoderm, I mentioned earlier in the course, the endoderm and the mesoderm.0578

So, important points here are that going from blastula to gastrula results in three germ layers: ectoderm, endoderm and mesoderm.0590

And this process requires the migration of some cells into the blastocele and the invagination or infolding of the blastula to form a gastrula.0602

Now, this structure in here is called an archenteron, and it is the primitive gut.0615

What you see here...let me put that down, primitive gut or GI tube, so primitive gut or primitive GI tube.0636

If you look at the archenteron, what you will see here is an opening called the blastopore.0645

Again, talking back about protostome versus deuterostome, what I am showing here is what would happen in a deuterostome.0657

In the deuterostome, the blastopore eventually forms the anus.0671

As this embryo continues to develop, this tube will reach one of these other sides, and there will be a second opening that forms the mouth.0682

So, in deuterostomes, the blastopore forms the anus, and then, there is a second opening that forms the mouth.0694

Protostomes work the other way. The blastopore forms the mouth, and then, the second opening forms the anus.0704

Now, to delve a little more into these three layers, you should be familiar with some of the structures that are formed from each of the germ layers.0729

So, starting with ectoderm, the ectoderm forms the outer layer, which is epidermis.0737

Epidermis is formed from ectoderm: hair, nails, the body coverings, the outer parts of the body.0749

Additionally, the nervous system is formed from the ectoderm.0761

So, it is not just the outer layer of the body. It is also the nervous system.0765

The cornea and the lens of the eye are formed from ectoderm as are pituitary glands.0769

These are some of the major structures formed from ectodermal tissue.0776

Next, the mesoderm, the mesoderm develops into the circulatory system: the muscles, the skeletal system, so the musculoskeletal system,0780

the excretory system and most of the reproductive system but not the germ cells but a lot of the reproductive system.0804

Endoderm: the linings of the GI tract and the respiratory tract are formed from endoderm also, accessory organs of digestion,0821

in addition, the thymus and the thyroid glands.0859

So, we have the linings of the GI and respiratory tract, the accessory organs of digestion meaning the pancreas, the liver and the gall bladder.0864

In addition, the thymus and the thyroid glands are formed from endoderm.0873

Alright, so, we talked about some of the major phases of early embryological development.0879

A zygote is formed. It undergoes cleavage.0884

Then, after cleavage, there is the formation of a ball of cells with the fluid-filled cavity called a blastula and then,0889

the formation of a 3-layered structure called a gastrula.0899

The next topic I am going to talk about is differentiation and how it is determined.0905

Like I said, ectoderm is going to become epidermis. Mesoderm is going to become circulatory system.0912

How is that determined which cells will become which? Well, one way is via what is called cytoplasmic determinants.0916

First, I am going to talk about non-mammalian cells, and the eggs of many animals such as amphibians have a polarity.0924

Mammal eggs do not have a polarity that is known to scientist. They may discover one, but right now, there is not obvious polarity,0933

but just talking now about eggs of many animals.0940

And I am going to focus, let's say, on amphibians right now, have a polarity as do many other types of eggs.0946

And this polarity is a result of different concentrations of materials in the cytoplasm of, I would say, differences in distribution of cytoplasmic materials.0957

These cells or some egg cells have two poles: an animal pole and a vegetal pole.0985

Taking the frog egg for example, well, vegetal pole is where a yolk is concentrated in general.1003

That is where the yolk is concentrated, and if you look at the example of a frog egg, about a third of the frog egg is taken up by a yolk.1011

Bird eggs have a lot of yolks, as well.1025

So, in the frog embryo, remember that the blastomere cells are going to end up uneven1028

because the presence of the yolk and the cleavage plane cannot pass through.1038

Now, one species that has been used for study has a darker coloration at the hemisphere that contains the animal pole.1042

And the vegetal hemisphere is more yellowish.1055

So, here, we have animal pole, and here, we have vegetal pole; and there is a color differentiation.1059

And at first, this light gray part would not be here right when the zygote forms, so, it would not look like that.1069

This would be covered. This would not be there.1075

It would just be these two hemispheres all the way across, so this part would not be here.1077

Now, upon fertilization, the animals hemisphere rotates towards the side of sperm penetration.1090

So, the sperm penetrates. The animal hemisphere, sort of, the covering of it rotates by where penetration has occurred.1099

And what this does is it reveals an area of cytoplasm called the gray crescent.1109

The membrane of the cell rotates and reveals this area called the gray crescent, and the gray crescent is crucial to the normal development of the embryo.1120

And classic experiments by Hans Spemann helped to demonstrate that what goes on in the cytoplasm,1133

what is found there, is important to development of particular cell types and organs.1140

So, Spemann in the 1920s did tests on some salamander eggs.1147

And what he was trying to figure out is if cytoplasmic factors were distributed unevenly in the fertilized egg.1157

So, what he did is he took fertilized eggs. He had a control group, and he had a test group.1166

And he used a string, very, very fine, and then, what he did is he divided the embryo at the two cell stage.1175

And he actually dissected the cell out, so now, there was...he used this along the normal cleavage plane, divided along the normal cleavage plane.1188

He separated those two cells, so he separated it out into two different cells so that he got two embryos, then, developing.1206

If this had stayed together, just one embryo would have developed.1217

But, he actually divided it, separated these two out so that they were not in contact anymore.1219

And then, what he ended up with is a normal embryo and another normal embryo.1225

In the test group, he actually divided it up so that one side got gray crescent- all the gray crescent.1233

The other side got no gray crescent, and in the normal group - excuse me - control group, both sides got gray crescent.1242

What he found was that the embryo derived from the cell at the 2-cell stage that got some gray crescent developed normally.1251

However, this embryo was missing all of its dorsal structures, and it did not complete development.1263

But, it got far enough that he could see that is was missing all dorsal structures.1271

From this, it could be concluded that cytoplasmic factors in the gray crescent were required for normal development of the dorsal structures.1283

Now, what are cytoplasmic determinants?1293

Well, cytoplasmic determinants are molecules like mRNA and protein that are found in different amounts/distributions in different parts of the fertilized egg.1296

And they are often transcription factors.1309

Therefore, these factors can influence the development of a cell by turning on a gene, then, a specific protein will be made.1321

And that can alter the cell structure and function.1331

The gray crescent must have some molecules in it or certain concentration of molecules that are needed for the development of these dorsal structures.1335

One thing to note in general is that cytoplasm is derived from the egg, so a sperm contributes DNA but not the cytoplasm or organelles.1343

Those are of maternal origin, which is why mitochondrial DNA is also derived from an individual's mother.1353

Both male and female offspring start out with the cytoplasm from their mother and also have the mitochondrial DNA from their mother.1360

Alright, now, we are going to talk about some specific types of embryo development, first looking at the development of the bird embryo.1373

So, I gave you just a very generalized discussion about blastula and gastrula formation.1379

And now, I just want to give you some examples to show you how there can be specific differences based on a species.1385

Because of the large amount of yolk that is present in a bird's egg, the cleavage furrow cannot cross the yolk.1394

So, development of the bird embryo is somewhat different than, say, in a mammalian embryo where there is barely any yolk.1403

Just starting out with cleavage, as I said, there is uneven cleavage1414

- so, just focusing on some differences between different species - due to the large amount of yolk.1419

Cytokinesis cannot be completed in the area where the egg yolk is, so as a result, only the sections that lack yolk are cleaved.1429

What happens is there ends up with egg cap of divided cells on top of the yolk.1443

So, the part of the fertilized egg that does not have yolk divides and divides, and it ends up being just as cap of cells sitting on the yolk.1454

And this forms a fluid-filled cavity between two layers of divided cells.1467

So, what it forms is similar to a blastocele. It forms two-layered structure with fluid-filled cavity.1476

And what this is called...there are different names for it, but one name and this overall structure,1495

the cell layers plus the fluid-filled cavity, is called a blastoderm or a blastodisc.1503

And this is really analogous to...it is equivalent to the blastula that we talked about.1516

And the fluid-filled cavity is same idea as a blastocele, so uneven cleavage results in a cap of dividing cells sitting on top of the yolk.1524

And those develop into a blastodisc, which has two layer of cells surrounding the fluid-filled cavity.1532

Now, these two layers of cells in the blastoderm are called the epiblast and the hypoblast, so the two layers: epiblast and hypoblast.1541

Those are the two layers of cells.1555

Now, recall that when we talked about gastrulation before, I talked about migration of cells into the blastocele that formation of the GI tube...1564

When I showed the tube, and I said, that start out with the blastula,1578

some cells migrate into the blastocele, eventually end up with the three-layered structure, the gastrula.1581

Well, that same type of migration does not occur in a bird embryo. Instead, what is called is a primitive streak forms.1587

And so epiblast cells migrate to form a primitive streak, and this is along the anterior, posterior axis of the embryo.1598

The embryo forms from the epiblast cells. Further migration will occur and differentiation to result in a three-layered gastrula.1626

And the gastrula structure is a bit different than what I talked about before.1641

Again, there are differences in each species, but the overall idea is the same. It is a three-layered structure.1645

So, migration, additional migration and differentiation to form three-layered gastrula.1651

And again, these germ layers are the same as the ones we have been talking about: ectoderm, the mesoderm and the endoderm.1666

So, a lot of the underlying points are similar starting out with cleavage eventually forming a two-layered structure with a fluid-filled center.1680

Here, it is called a blastoderm.1689

Now, we have cell migration, this time, to form a primitive streak and further migration differentiation1691

resulting in a three-layered gastrula with an ectoderm, endoderm and mesoderm layer.1699

Now, before we go on to talk about mammalian embryos, I want to talk about the topic of extraembryonic membranes.1705

Recall in diversity of life, I mentioned that animals can be classed as amniotes. Some animals are called amniotes.1714

And in amniotes, the embryo is surrounded by a series of membranes.1722

It gets its name from the amnion, and the amnion contains amniotic fluid that cushions the embryo.1731

In mammals, the membranes in the embryo are all found inside the uterus of the mother.1738

Whereas in birds and reptiles, the embryo along with its extraembryonic membranes are found within an egg.1744

Now, the development within an egg or within a uterus is what allowed for amniotes1752

to live very successfully on land and reproduce in a terrestrial environment.1759

They are not tied to water the way amphibians are.1763

And essentially, what has been done is that amniotes have created an aqueous1767

environment for the developing embryo within either an egg or within the uterus.1772

So, I am going to focus here. This is a bird egg with a developing bird embryo, but a lot of the basic points are the same for mammals.1777

There are differences. There are differences, but just the basic idea of having this series of membranes.1788

Starting out with...we are just focusing on a bird here, so there is a shell.1795

Now, the layer, the chorion, is the extraembryonic membrane that surrounds the other membranes and the embryo itself.1802

So, it is found just inside the shell, and it functions in gas exchange.1813

So, oxygen can diffuse into the shell, cross the chorion, diffuse across the chorion, and CO₂ is going to leave.1818

Gas exchange occurs. The chorion plays a role in gas exchange.1830

Next is the yolk sac. It contains yolk, which has nutrients for the developing embryo.1837

Now, in mammals, there is just a fluid inside the yolk sac.1850

There is not a yolk because the placenta and the umbilical cord take on roles that some of these membranes have in a bird or a reptile.1855

The next structure is the allantois. The allantois functions in gas exchange, as well.1866

So, it has a role in gas exchange as does a chorion, but it also is important for waste storage specifically, uric acid storage.1880

Remember that in birds, the nitrogenous waste is uric acid.1890

Again, in mammals, there is the placenta. There is the umbilical cord, and there is this exchange.1895

We end up with the mother's body that allows for waste to be removed and nutrients to be given, which is much different situation than an egg.1903

But again, some of these basics do apply to both groups of animals.1911

Finally, the amnion: the amnion is a sac that surrounds the embryo, and it contains amniotic fluid.1917

The amniotic fluid protects and cushions the embryo, so let's go on now and talk about the mammalian embryo.1925

After fertilization in the fallopian tube, the zygote is going to travel into the uterus where it implants.1941

So, cleavage, because there is not all this yolk in the mammalian eggs, then, the fertilized egg, the cleavage is holoblastic.1949

The result is that there are evenly-sized blastomeres.1959

In mammals, with the formation of the hollow ball filled with fluid, the structure is called the blastocyst.1965

This is the mammalian version of the blastula, hollow ball with a blastocele inside, so it is a hollow ball of cells filled with fluid.1976

There is another structure you should be familiar with. It is called an inner cell mass.1999

This is a cluster of cells that will form an embryo and the extraembryonic membranes like the amnion and the chorion.2005

As we just discussed with the bird embryo, there are two layers of cells, and these are initially two layers of cells: the epiblast and the hypoblast.2032

The epiblast is actually the upper layer in the blastocyst, so an upper layer of cells, and the lower layer of cells is the hypoblast.2047

The embryo forms from the epiblast.2064

Now, as I mentioned, in mammals, there is implantation into the uterine wall, and it occurs due to a structure called the trophoblast.2071

Epithelial cells in the outer part of the blastocyst produce enzymes that allow for implantation.2085

So, this is an outer layer of cells in the blastocyst as it says here.2092

And what these cells do is they produce enzymes, then, the embryo can implant into the uterine wall.2102

Implantation occurs about 7 days, so about a week after fertilization, and cells from the trophoblast, sort of, invade the endometrium.2108

And they encounter blood vessels because the endometrium is very well-vascularized, and eventually, a placenta will form.2118

And it is partly from the trophoblast as well as partly from the endometrial wall.2126

And the placenta has extremely important functions for a mammalian embryo as does the umbilical cord.2131

And those structures allow for the embryo to receive nutrients and to get rid of waste.2139

Following implantation, the gastrula forms.2150

And there is also in mammals a primitive streak that forms from epiblast cells, so they are migration of epiblast cells.2166

Eventually, this will become the mesoderm and the endoderm.2178

So, the result is going to be a three-layered gastrula surrounded by extraembryonic membrane.2181

So, we get a holoblastic cleavage followed by blastocyst formation. This is like a blastula.2188

There is an inner cell mass, and this will form the embryo and the extraembryonic membranes.2194

Following implantation, which is facilitated by the trophoblast, the gastrula will form.2201

And again, the gastrula has the three layers: ectoderm, endoderm, mesoderm.2211

Alright, so, we have talked about early embryonic development. Now, I would like to focus a little on organogenesis, which is the form.2216

So, organogenesis, as I said, is the formation of organs, and during this process, the cells differentiate.2226

During this differentiation, groups of cells form folds.2234

They also form clusters. These clusters are called condensations, or they may split.2240

So, they can fold over, for example, to form a tube structure as I am going to talk about in a minute.2251

Now, the notochord and the neural tube are some of the first structures to form in chordates.2256

So, I will focus on that as we talk about organogenesis, and I want to explain the color coding here.2263

Blue is ectoderm. Red is the mesoderm, and yellow is ectoderm.2270

So, just to briefly review from our discussion in the diversity of life about chordates, remember that notochord means back string.2285

And it is a flexible rod-type structure that is located dorsal to or above the GI tube, so here, we have the GI tube.2294

This a cross-section showing the formation of the notochord, and this is the GI tube.2305

Now, remember that in adult chordates, usually, the notochord is gone. It is there during embryological development, at some point in all chordates.2310

But, for vertebrate chordates, often the notochord is not present in the adult.2318

Whereas, in invertebrate chordates, since they lack a backbone, the notochord can end up providing support.2324

But, for vertebrates, it is usually just...there are vestiges of it left.2332

What we have right here is the notochord, and ectoderm...remember, blue is the ectoderm.2339

It forms from just above the notochord. The neural tube forms from ectoderm.2352

Let me show that right here as a tube formed out of ectoderm, and this is the neural tube.2361

The CNS/central nervous system, the brain and the spinal cord, will eventually develop from the neural tube.2373

In addition, near the neural tube are what is called neural crest cells.2380

These are located near the edges of the neural tube, and the peripheral nervous system, so, peripheral nerves, will form from this.2388

As will other structures like teeth form from these cells.2398

So, this just shows the very beginnings of organogenesis focusing on the way it would happen2403

in chordates with the development of the notochord and the neural tube very early on.2410

Now, when we talked about differentiation, I mentioned cytoplasmic factors, cytoplasmic determinants, which influence the fate of cells.2416

Differential distribution of molecules in the cytoplasm results in cells that it could be different types.2426

And once cleavage occurs, then, you might have a cell with a different amount of cytoplasmic factors than another one.2434

Now, there is a second mechanism through which cells differentiate from one another, and this is via induction.2439

Induction refers to the influence of one group of cells on the development of another group of cells.2448

This occurs, this is mediated because of diffusible factors, so one group of cells can secrete molecules that can, then, affect other nearby cells.2455

Now, induction could also occur via cell to cell interaction.2470

One group of cells could actually be physically touching another group of cells and affect it.2477

I am mentioning diffusible factors, but keep in mind, cell to cell interactions may also play a role.2481

One group of cells might secrete certain molecules that influence another group of cells to2489

start making particular proteins that might in turn establish that other group of cells' fate.2494

The concept of the fate mapping was very important in getting us as far as we have gotten in understanding embryology.2502

And this is a technique that researchers had used to learn about embryological development.2510

So, in fate mapping, what happens is a researcher studies a group of cells or a particular cell and follows it through development,2517

so follows it through cleavage and morula formation, blastula and gastrula and then, eventually, sees what type of organ it becomes.2537

In other words, what is that cell's fate.2551

So, by seeing that a certain type of cell or certain group of cells eventually becomes nervous system2554

versus another group of cells in a certain location becomes circulatory system, so that is fate mapping.2561

OK, we are going to now review development with four examples starting with example one.2569

Place the following processes and stages of embryological development in the order in which they occur.2576

We have five stages: gastrula, zygote, fertilization, cleavage and blastula.2583

And just to note, gastrula formation is called gastrulation also, and blastula formation, you might hear it called blastulation.2593

OK, so, the first event is fertilization.2600

And upon fertilization, the union of the sperm and egg, the zygote is formed- the diploid single-celled very early embryo.2606

The zygote, then, undergoes rapid cell division, and this is called cleavage.2619

So, there is very little growth in the overall size of the embryo, but it has been subdivided into blastomeres.2628

Next, with continued cell division and formation of a fluid-filled cavity, you will end up hollow ball cells called the blastula.2636

Finally, reorganization, further cell division and differentiation forms a three-layered structure called a gastrula.2648

So, the correct order is fertilization, zygote, cleavage, blastula and gastrula.2657

OK, in example two: in an experiment performed by Hans Spemann and Hilde Mangold in the 1920s, cells from the dorsal lip of an embryo2674

from one newt species were transplanted to the ventral side of the gastrula of a recipient embryo from another newt species.2686

So, let's stop and look at what is going on.2695

Cells were taken from the dorsal lip from one newt species, and they were transplanted to the ventral side of a recipient embryo,2698

so, cells from the dorsal side of one embryo transplanted to the ventral side of a recipient embryo.2710

The transplanted tissue developed into a second notochord in the recipient embryo.2720

So, the recipient embryo had a regular notochord as expected and a second notochord.2725

Neural folds also developed near the second notochord, so there were two sets of notochord, two sets of neural folds.2733

These tissue were found to be composed of recipient cells.2741

Donor tissue was transplanted to the other embryo, but that second set of structures actually formed from recipient tissue.2747

Through what mechanism could the transplanted cells have altered the fate of the recipient cell?2758

So, somehow, putting the dorsal cells onto a recipient embryo caused the recipient cells to form these dorsal structures. What could be the mechanism?2763

Well, remember, we talked about induction, and induction is when one group of cells influences the development of another group of cells.2777

And it can be mediated by diffusible factors, so that is a possible mechanism for what occurred here.2786

Example three: state which germ layer ectoderm, endoderm or mesoderm, each of the fine systems and organs develops from.2802

Pancreas: well, the pancreas develops from endoderm tissue.2812

Next, we have the nervous system. That originates from ectodermal tissue.2823

Skeletal system originates from mesoderm, and finally, the lining of the respiratory tract develops from endoderm tissue.2833

Describe the function of each of the fine extra embryonic membranes.2852

First, yolk sac: well, it contains yolk, and what does the yolk do? It provides nutrition for the developing embryo.2859

Next, we have the amnion. This is the sac that surrounds the embryo, and recall that it contains amniotic fluid.2876

And the amniotic fluid cushions and protects the embryo or fetus.2891

Allantoise: the allantoise functions in gas exchange partly.2904

The chorion does, too, but this also has a role in gas exchange and also stores waste products for example, uric acid, which is nitrogenous waste.2911

In the case of the bird embryo...so, I will just say "stores waste products".2930

Remember that extra embryonic membranes have a little bit different function in different animals.2938

Here, mentioning uric acid, that would apply to a bird, but in general, it stores waste products for an embryo that would2945

need to store it that could not eliminate the waste products some other way such as a mammalian embryo could.2955

OK, next, the chorion: it surrounds the other membranes, and it has a function in gas exchange.2962

Again, in mammals, some of these functions are taken over by the umbilical cord and the placenta.2976

But just talking in general about functions for example in mammals the yolk sac, generally, it just contains a clear fluid.2982

So, this is more focused on what would happen in amniotes such as a bird versus a mammal, OK?2990

That concludes this lesson on development at Educator.com.2999

Thanks for visiting.3004