And now I want to talk about the amniotic egg. You'll remember that I told you when I wastalking about the invasion of land, that when we look at vertebrate animals what we see is thatthere are no examples of vertebrate animals that have a very, very tiny free-living, what you mightcall a larval stage, but it's probably not strictly correct to call it that, but similar to a tiny fish.
Fish can produce very tiny eggs. And a little couple of a millimeter long fish can hatch outand can find food and survive in the aquatic environment and grow up to be a great big fish. Wedon't see any examples of that among vertebrate animals.
What we see instead is an alternate solution. And that is the evolution of this amnioticegg. Which is an egg that if you really look at it in the big sense of the thing, what this egg isdoing is allowing vertebrate animals to produce larger newborn young than they would be able toproduce otherwise. That's really what this egg does. It allows vertebrates to produce a largernewborn young.
If you take a look at the largest fish or amphibian eggs, they are in the neighborhood ofmaybe 4 or 5 millimeters in diameter. You know, a salmon egg, for example, you might use forfishing is less than a centimeter in diameter in most cases. Whereas an amniotic eggs can gettingto be really truly huge.
An ostrich produces the largest amniotic egg of any living vertebrate. And those canweigh one and a half kilograms. That's the size of 30 chicken eggs.
But there are even fossil eggs of a couple of other species of birds that were about threetimes as big as the ostrich. Elephant birds, for example, I have seen them and the eggs are like thisbig. And that's tremendously larger than you would ever be able to get from any amphibian egg orfish egg.
And the reason is that the amniotic egg satisfies some basic requirements of any livingorganism in a different way than amphibian and fish eggs.
If we were to just sit here and say what are the most fundamental essential requirementsfor life for a living organism, whether we're talking about a vertebrate or invertebrate, terrestrial,or an aquatic, adult, embryonic. We would come up with some real fundamental things. Water.Life is fundamentally an aqueous process, a very complicated series of chemical reactions takingplace in the aqueous phase.
Nutrition. You have to have some food source. Of course, plants make their own.Animals get them from eating other animals or plants. You have to have some form of nutrition.
There are going to be some waste products, solid waste products or aquatic wasteproducts that result from metabolism of your nutrients. And there is going to be a need foroxygen and carbon dioxide, for an exchange of respiratory gases.
Now, these basic needs are going to be true for all organisms. And in order to survive anorganism is going to have to be able to satisfy these basic needs.
When we look at a fish egg, for example, or an amphibian egg, what we will see is thatthese eggs, which are -- I'm using these because these are representative of the ancestors fromwhich the amniotic egg evolved, these eggs have one structure which is called a extra embryonicmembrane that provides for nutrition.
So we're going to look at these extra embryonic membranes. In this case, we don't mean aphospholipid bilayer membrane. We use that term "membrane" in a number of different ways inbiology. In this case we're talking about a multi-cellular vascularized structure that grows out ofthe embryo. That's what extra embryonic means, it's outside of the embryo.
And in a fish egg there is only one extra embryonic membrane. And it is there to providefor nutrition, and it's called a yoke sac. And you looked at the yoke that's included in some of theeggs of a frog. You looked that in the section of a lab where you were looking at sections offrogs. But if you see an embryonic shark or embryonic larger type of fish, it will actually have astructure growing out of their belly, called a yoke sac, that surrounds of bunch of yoke andabsorbs the nutrients out of the yoke. That's what yoke is, it's a bunch of fat and protein that fuelsdevelopment.
However, these other requirements in the case of a fish or amphibian egg are met purelyby diffusion, because the fish and amphibian eggs are going to be developing -- in most cases aregoing to be developing in the water. So there are surrounded by water. They don't need to have amembrane, they can do that by diffusion.
Waste products are just going to diffuse out of the embryo into the abundant supply ofwater that is surrounds the embryo. And the respiratory gases are also going to be obtained bydiffusion.
However, one thing that we know about diffusion is that the amount of substancediffusing per unit time, which we would symbolize by the symbol "Q dot" (?), that means theamount moving per unit of time. It might be moles per second. The amount that is diffusing perunit of time is inversely proportional to the distance that it has to move.
That little alpha symbol means proportional. That is the greater the distance thatsomething has to diffuse, the more slowly it moves. Everything else being constant. So thisfeature of diffusion limits the size that you can be and still have diffusion be your means ofobtaining water, respiratory gases, and waste products. If you are going to be very big, thediffusion distances are going to be so big that you cannot have this exchange occur by diffusion.
So what the amniotic egg does, is it has 3 new structures that provide for these other 3means of exchange. So in an amniotic egg, the water is provided by one of the extra embryonicmembranes, which is called the "amnion." This is the membrane that gives the egg its name. Theegg is named after this membrane.
There is a membrane called the amnion and it surrounds the embryo. If we were to makea diagram of amniotic egg -- this is a diagram that you probably want to copy down and learn,because sometimes it can appear on the exam.
So this is a diagramatic representation of an amniotic egg. We have an embryo with bighead, limbs, tail, couple of 4 limbs and hind limbs. That's the embryo. There is bunch of yoke. Sothis might be like a bird egg sort of halfway through development. And it has this yoke sac whichhas grown out of the digestive tract on the animal and completely surrounded the yoke.
It has a very extensive blood supply. It has lots of arteries and veins and capillaries. Andthe yoke sac in the amniotic egg is going to be providing for the nutritional requirements of theembryo. But in a sense that's a -- if we look at it from an evolutionary sense comparingamphibians with reptiles, this is an old structure. This is a structure that the reptiles inherited fromtheir amphibian ancestors. So it's not one of the new extra embryonic membranes.
But the amnion is a new one, and it is the membrane that surrounds the embryo. So in thissame diagram the amnion would be here like this.
And it's a very thin transparent membrane, but it completely surrounds the embryo. Itactually grows out of the embryo. How it gets to this position is complicated to explain. So wewon't worry about it.
But by about halfway through development, the amnion has completely surrounded theembryo. And it contains what is sometimes called the -- or sort of the flowery way is referred toas the primitive pond. It contains the amniotic fluid. And this amniotic fluid is the water supply forthe embryo.
That same fluid, by the way, you have probably heard about it when there is genetictesting or concern about the status of a human developing embryo, the doctors will stick a needleinto the belly through the wall of the uterus and take a sample of this fluid. And that is calledamniocentesis.
It's taking a sample of this amniotic fluid, which because it completely bathes the embryo,it has embryonic cells in it. They'll do this process, an amniocentesis, take a bit of this amnioticfluid out and then they'll culture the cells and they can test for genetic difficulties that may beoccurring in the embryo.
So that's the first new extra embryonic membrane, it's the amnion, and it's function is toprovide for the water requirements of the embryo.
Another new extra embryonic membrane is called the "Allantois." And the allantoisdevelops out of the cloaca of the animal. So it attached back here, and it can become a very largemembrane. And it's a sac-like structure. It's a blind sac. It's open right to the cloaca, and itcontains the allantoic fluid, which contains the urine of the animal.
So this embryo has functioning kidneys. And the kidneys are producing urine. And theurine flows into this allantois, which is a sac-like structure. The fluid is called the allantoic fluid,but it is the pee of the animal. And it's going to have -- among other things, it's going to uricacid.
So after a bird egg or a reptile egg hatches, it leaves the allantois behind. You can see alittle deposit of that white gooey uric acid that is deposited in the allantois. The so the allantois isa second new extra embryonic membrane, and its function is to provide a place for the storage ofwaste products in the amniotic egg.
Now, there is a third new extra embryonic membrane. And there is third requirement.And you won't be surprised to learn that there is not an exact correspondence between these two.Nature is not quite that cooperative.
The third extra embryonic membrane is called the chorion. So you need to learn to makethis distinction. There are 3 new extra embryonic membranes in the amniotic egg: The amnion, theallantois, and chorion.
And there are 3 requirements that are met by these extra embryonic membranes. But thelast one, the requirement for respiratory gasses is not met by the chorion alone. It's met by acompound structure formed by the fusion of the chorion and the allantois. And it's called thechorioallantoic membrane.
The chorion itself develops out of the embryo and comes to line the inside of the egg, likethat. That's the chorion. But where the allantois and the chorion fuse, that is where the allantoisgrows out and touches the chorion. That's where we have this chorioallantoic membrane, and thatis the embryos respiratory structure. It serves the same purpose as the lungs of the embryo.
Now, if you read in zoology text books about the amniotic egg, there is a great deal ofimportance paid to this egg, because this is an invention that occurred in the reptiles -- remember,I explained to you the logic for why we believe that the captorhinomorphs were the first animalsto have this, because the fossils of amniotic eggs are found from the same geological deposits,they are the same age, and because the fact that all of descendants of the captorhinomorphsproduce amniotic eggs, there is a lot of attention paid to it.
But there are some statements that are made that are simply untrue. One of thosestatements is that the amniotic egg allowed vertebrates to invade land. And that's not true.
There are vertebrates that live in the terrestrial environment and do not produce anamniotic egg. The labyrinthodonts, a fossil group of amphibians, where some of them are veryhighly terrestrial. And they simply went back to water to reproduce. There are also frogs that livein the desert out near Yuma, Arizona, and have been living there for tens of thousands of years.And they don't have to go to San Diego to have babies.
They have their babies in the sand dunes and outside Yuma, Arizona, and they are quitethoroughly terrestrial. And they reproduce when the winter rains produce big ponds of water.
I may be splitting a point here, but I think it's only fair acknowledging the fact that thisdid not really enable vertebrates to invade land.
Another term you will encounter sometimes, this is referred to as "cleidoic" egg. Cleidoicis a Latin term that means a "sealed box." The suggestion is that this egg is sealed off from thesurrounding environment. And that is absolutely not true either.
Not only is there an exchange of oxygen and carbon dioxide between the embryo and theenvironment, but there is also the exchange of water. Bird eggs lose water. Bird eggs need to loseabout 15 percent of their initial weight in the form of water or the embryo isn't going to hatch.
So that's not a cleidoic egg if it has to lose water. And reptile eggs must take up water.Reptile eggs have to soak up water from the soil in which they are buried in order to successfullydevelop. If they don't, they won't hatch.
So this egg really is not a sealed box. It's not a cleidoic egg; it does exchange nutrientswith the environment. But I think it's fair to say that it is one of the major evolutionary steps in thehistory of the vertebrate. Certainly the great diversity of the terrestrial vertebrates that we seewhen you look at all of the extinct reptiles of the Mesozoic, all the dinosaurs and Euryapsids, allof the birds, and all the mammals.
Mammals also produce an amniotic egg. Even viviparous mammals such as yourself stillhave these same basic structures surrounding the embryo. The placenta, for example, is astructure that is similar to the chorioallantoic membrane. It's serving the same purpose. It hasblood vessels and it serves as a site of exchange of respiratory gases.