120 FROM FISH TO PHILOSOPHER 



Romer says, 'it was the egg which came ashore first; the 

 adult followed later/ 



The first great invention of the 'reptiles' (we use the word 

 with reservations at this time) was the 'amniotic' egg, 

 so-named because it contains a closed sac or 'amnion' 

 which is fiUed with fluid and which suppHes an artificial 

 aquatic environment in which the embryo can develop 

 through the larval stage into a miniature but adult ani- 

 mal. (Figure 9.) A second membrane, the 'allantois,' 

 creates a cavity (the allantoic cavity) which communi- 

 cates with the embryonic kidney and serves as a reposi- 

 tory for such urine as is formed. A third membrane, the 

 'chorion,* initially fining the eggshell, imites with the al- 

 lantoic membrane to form a respiratory mantle, richly 

 perfused by blood vessels and serving to take in oxygen 

 and give off carbon dioxide for the developing embryo. 

 A yolk, greatly enlarged beyond that of the fishes or 

 Amphibia, supplies food for sustained development. 

 Such is a hen's egg, which is not significantly different 

 from that of the reptiles. The quantity of yolk in the eggs 

 of the Amphibia and of the fishes varies widely from 

 one species to another, and the enlargement of the yolk 

 sac in the amniotic egg represents merely a quantitative 

 variation, while the chorion can be considered to be an 

 elaboration of the primitive egg membrane. The amni- 

 otic membrane, however, with its artificial internal en- 

 vironment for the embryo, is a novel invention which has 

 the appearance of a most happy accident. 



These three embryonic patents, the amniotic, the al- 

 lantoic, and the chorionic membranes, are practically 

 identical in structure and function in the birds and rep- 

 tiles and persist with greatly elaborated fimction in the 

 mammals, the amnion here supplying the fetal mem- 

 brane filled with amniotic fluid, the chorion becoming 

 the fetal contribution to the placenta, the allantois part 

 of the umbilical cord. In the reptiles, birds and two prim- 



