And now I want to talk about the amniotic egg. You'll rememberthat I told you when I was talking about the invasion of land,that when we look at vertebrate animals what we see is that thereare no examples of vertebrate animals that have a very, very tinyfree-living, what you might call a larval stage, but it'sprobably not strictly correct to call it that, but similar to atiny fish.
Fish can produce very tiny eggs. And a little couple of amillimeter long fish can hatch out and can find food and survivein 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 theevolution of this amniotic egg. Which is an egg that if youreally look at it in the big sense of the thing, what this egg isdoing is allowing vertebrate animals to produce larger newbornyoung than they would be able to produce otherwise. That's reallywhat this egg does. It allows vertebrates to produce a largernewborn young.
If you take a look at the largest fish or amphibian eggs, theyare in the neighborhood of maybe 4 or 5 millimeters in diameter.You know, a salmon egg, for example, you might use for fishing isless than a centimeter in diameter in most cases. Whereas anamniotic eggs can getting to be really truly huge.
An ostrich produces the largest amniotic egg of any livingvertebrate. And those can weigh one and a half kilograms. That'sthe size of 30 chicken eggs.
But there are even fossil eggs of a couple of other species ofbirds that were about three times as big as the ostrich. Elephantbirds, for example, I have seen them and the eggs are like thisbig. And that's tremendously larger than you would ever be ableto get from any amphibian egg or fish egg.
And the reason is that the amniotic egg satisfies some basicrequirements of any living organism in a different way thanamphibian and fish eggs.
If we were to just sit here and say what are the mostfundamental essential requirements for life for a livingorganism, whether we're talking about a vertebrate orinvertebrate, terrestrial, or an aquatic, adult, embryonic. Wewould come up with some real fundamental things. Water. Life isfundamentally an aqueous process, a very complicated series ofchemical reactions taking place in the aqueous phase.
Nutrition. You have to have some food source. Of course,plants make their own. Animals get them from eating other animalsor plants. You have to have some form of nutrition.
There are going to be some waste products, solid wasteproducts or aquatic waste products that result from metabolism ofyour nutrients. And there is going to be a need for oxygen andcarbon dioxide, for an exchange of respiratory gases.
Now, these basic needs are going to be true for all organisms.And in order to survive an organism is going to have to be ableto satisfy these basic needs.
When we look at a fish egg, for example, or an amphibian egg,what we will see is that these eggs, which are -- I'm using thesebecause these are representative of the ancestors from which theamniotic egg evolved, these eggs have one structure which iscalled a extra embryonic membrane that provides for nutrition.
So we're going to look at these extra embryonic membranes. Inthis case, we don't mean a phospholipid bilayer membrane. We usethat term "membrane" in a number of different ways inbiology. In this case we're talking about a multi-cellularvascularized structure that grows out of the embryo. That's whatextra 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 provide for nutrition, and it's called a yokesac. 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 youwere looking at sections of frogs. But if you see an embryonicshark or embryonic larger type of fish, it will actually have astructure growing out of their belly, called a yoke sac, thatsurrounds of bunch of yoke and absorbs the nutrients out of theyoke. That's what yoke is, it's a bunch of fat and protein thatfuels development.
However, these other requirements in the case of a fish oramphibian egg are met purely by diffusion, because the fish andamphibian eggs are going to be developing -- in most cases aregoing to be developing in the water. So there are surrounded bywater. They don't need to have a membrane, they can do that bydiffusion.
Waste products are just going to diffuse out of the embryointo the abundant supply of water 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 theamount of substance diffusing per unit time, which we wouldsymbolize by the symbol "Q dot" (?), that means theamount moving per unit of time. It might be moles per second. Theamount that is diffusing per unit of time is inverselyproportional to the distance that it has to move.
That little alpha symbol means proportional. That is thegreater the distance that something has to diffuse, the moreslowly it moves. Everything else being constant. So this featureof diffusion limits the size that you can be and still havediffusion be your means of obtaining 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 havethis exchange occur by diffusion.
So what the amniotic egg does, is it has 3 new structures thatprovide for these other 3 means of exchange. So in an amnioticegg, the water is provided by one of the extra embryonicmembranes, which is called the "amnion." This is themembrane that gives the egg its name. The egg is named after thismembrane.
There is a membrane called the amnion and it surrounds theembryo. If we were to make a diagram of amniotic egg -- this is adiagram that you probably want to copy down and learn, becausesometimes it can appear on the exam.
So this is a diagramatic representation of an amniotic egg. Wehave an embryo with big head, limbs, tail, couple of 4 limbs andhind limbs. That's the embryo. There is bunch of yoke. So thismight be like a bird egg sort of halfway through development. Andit has this yoke sac which has grown out of the digestive tracton the animal and completely surrounded the yoke.
It has a very extensive blood supply. It has lots of arteriesand veins and capillaries. And the yoke sac in the amniotic eggis going to be providing for the nutritional requirements of theembryo. But in a sense that's a -- if we look at it from anevolutionary sense comparing amphibians with reptiles, this is anold structure. This is a structure that the reptiles inheritedfrom their amphibian ancestors. So it's not one of the new extraembryonic membranes.
But the amnion is a new one, and it is the membrane thatsurrounds the embryo. So in this same diagram the amnion would behere like this.
And it's a very thin transparent membrane, but it completelysurrounds the embryo. It actually grows out of the embryo. How itgets to this position is complicated to explain. So we won'tworry about it.
But by about halfway through development, the amnion hascompletely surrounded the embryo. And it contains what issometimes called the -- or sort of the flowery way is referred toas the primitive pond. It contains the amniotic fluid. And thisamniotic fluid is the water supply for the embryo.
That same fluid, by the way, you have probably heard about itwhen there is genetic testing or concern about the status of ahuman developing embryo, the doctors will stick a needle into thebelly through the wall of the uterus and take a sample of thisfluid. And that is called amniocentesis.
It's taking a sample of this amniotic fluid, which because itcompletely bathes the embryo, it has embryonic cells in it.They'll do this process, an amniocentesis, take a bit of thisamniotic fluid out and then they'll culture the cells and theycan test for genetic difficulties that may be occurring in theembryo.
So that's the first new extra embryonic membrane, it's theamnion, and it's function is to provide for the waterrequirements of the embryo.
Another new extra embryonic membrane is called the"Allantois." And the allantois develops out of thecloaca of the animal. So it attached back here, and it can becomea very large membrane. And it's a sac-like structure. It's ablind sac. It's open right to the cloaca, and it contains theallantoic fluid, which contains the urine of the animal.
So this embryo has functioning kidneys. And the kidneys areproducing urine. And the urine flows into this allantois, whichis 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 -- amongother things, it's going to uric acid.
So after a bird egg or a reptile egg hatches, it leaves theallantois behind. You can see a little deposit of that whitegooey uric acid that is deposited in the allantois. The so theallantois is a second new extra embryonic membrane, and itsfunction is to provide a place for the storage of waste productsin the amniotic egg.
Now, there is a third new extra embryonic membrane. And thereis third requirement. And you won't be surprised to learn thatthere is not an exact correspondence between these two. Nature isnot quite that cooperative.
The third extra embryonic membrane is called the chorion. Soyou need to learn to make this distinction. There are 3 new extraembryonic membranes in the amniotic egg: The amnion, theallantois, and chorion.
And there are 3 requirements that are met by these extraembryonic membranes. But the last one, the requirement forrespiratory gasses is not met by the chorion alone. It's met by acompound structure formed by the fusion of the chorion and theallantois. And it's called the chorioallantoic membrane.
The chorion itself develops out of the embryo and comes toline the inside of the egg, like that. That's the chorion. Butwhere the allantois and the chorion fuse, that is where theallantois grows out and touches the chorion. That's where we havethis chorioallantoic membrane, and that is the embryosrespiratory structure. It serves the same purpose as the lungs ofthe embryo.
Now, if you read in zoology text books about the amniotic egg,there is a great deal of importance paid to this egg, becausethis is an invention that occurred in the reptiles -- remember, Iexplained to you the logic for why we believe that thecaptorhinomorphs were the first animals to have this, because thefossils of amniotic eggs are found from the same geologicaldeposits, they are the same age, and because the fact that all ofdescendants of the captorhinomorphs produce amniotic eggs, thereis a lot of attention paid to it.
But there are some statements that are made that are simplyuntrue. One of those statements is that the amniotic egg allowedvertebrates to invade land. And that's not true.
There are vertebrates that live in the terrestrial environmentand do not produce an amniotic egg. The labyrinthodonts, a fossilgroup of amphibians, where some of them are very highlyterrestrial. And they simply went back to water to reproduce.There are also frogs that live in the desert out near Yuma,Arizona, and have been living there for tens of thousands ofyears. 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 quite thoroughly terrestrial. And theyreproduce when the winter rains produce big ponds of water.
I may be splitting a point here, but I think it's only fairacknowledging the fact that this did not really enablevertebrates to invade land.
Another term you will encounter sometimes, this is referred toas "cleidoic" egg. Cleidoic is a Latin term that meansa "sealed box." The suggestion is that this egg issealed off from the surrounding environment. And that isabsolutely not true either.
Not only is there an exchange of oxygen and carbon dioxidebetween the embryo and the environment, but there is also theexchange of water. Bird eggs lose water. Bird eggs need to loseabout 15 percent of their initial weight in the form of water orthe embryo isn't going to hatch.
So that's not a cleidoic egg if it has to lose water. Andreptile eggs must take up water. Reptile eggs have to soak upwater from the soil in which they are buried in order tosuccessfully develop. If they don't, they won't hatch.
So this egg really is not a sealed box. It's not a cleidoicegg; it does exchange nutrients with the environment. But I thinkit's fair to say that it is one of the major evolutionary stepsin the history of the vertebrate. Certainly the great diversityof the terrestrial vertebrates that we see when you look at allof the extinct reptiles of the Mesozoic, all the dinosaurs andEuryapsids, all of the birds, and all the mammals.
Mammals also produce an amniotic egg. Even viviparous mammalssuch as yourself still have these same basic structuressurrounding the embryo. The placenta, for example, is a structurethat is similar to the chorioallantoic membrane. It's serving thesame purpose. It has blood vessels and it serves as a site ofexchange of respiratory gases.