122 COMPARATIVE ANATOMY 



which impart contractility to the amnion. Its gentle undulations keep 

 the amnionic fluid moving, thus preventing local stagnation in the fluid 

 and possible adhesions of embryonic parts. 



The main importance of the chorion is in its serving, in conjunction 

 with the allantois (rarely the yolk-sac), to form respiratory, excretory 

 and nutritive membranes. 



The allantois in reptiles and birds is the embryonic respiratory organ. 

 In viviparous reptiles its circulation may pick up some nutrient material 

 from the uterine wall or fluids. The cavity of the allantois serves as 

 receptacle for waste from the embryonic kidneys. Consistently with 

 this function its proximal portion may persist, after hatching, as the 

 urinary bladder. 



In mammals, although primitively the yolk-sac may have had placental 

 possibilities, the allantois came to be the all-important agency in estab- 

 lishing the placenta. The placental villi are primarily chorionic but their 

 vascular structures are usually derived from the allantoic circulatory 

 system. In the placenta, then, are concentrated the vital functions, 

 nutrition, respiration and, at least in part, excretion. 



The vascular splanchnopleural wall of the yolk-sac is the essential 

 nutritive membrane in reptiles, birds and primitive mammals, but 

 becomes rudimentary in later mammals. 



SUMMARY AND INTERPRETATION 



A backward glance over the foregoing account of vertebrate develop- 

 ment suggests emphasis on the following points. 



Cleavage may be described as a process of mobilizing- the agencies 

 within the fertilized egg. The smallest egg is a large cell. Nutrition, 

 respiration and excretion involve the limiting surfaces of cells. Metabolic 

 rate is high during development. Cleavage increases cell surface. 

 Nuclear material is certainly somehow important. Cleavage increases 

 the quantity of nuclear material both absolutely and relatively to the 

 amount of cytoplasm. 



In the blastula stage the embryonic material becomes arranged in 

 form of a layer. The increased exposure of surface to the external medium 

 favors metabolism. The disposition of cells in a layer is especially 

 significant in view of the fact that the adult animal is constituted essen- 

 tially of layers of cells. 



Gastrulation transforms the one layer of the blastula into two, an 

 outer and an inner. The cavity of the blastula has no permanent signifi- 

 cance. The new cavity produced by gastrulation is the definitive enteron. 

 The ectoderm, potentially protective and nervous, provides for necessary 

 relations of an animal to its environment; the endoderm is nutritive. At 

 the gastrula stage, therefore, a minimum metazoan animal is estabhshed. 



