290 Cell as Basis of Organic Activity 



EMBRYONIC MEMBRANES 



Among the vertebrates there are four separate embryonic mem- 

 branes : the yolk sac, the amnion, the chorion, and the allantois. The 

 yolk sac is found in all vertebrates, but is of particular importance in 

 those possessing large amounts of yolk. The last three membranes are 

 found only in the reptiles, birds, and mammals. They represent part 

 of the adaptation of these animals toward a land life. These membranes 

 give the developing young protection against desiccation and injury. 

 The reptiles, birds, and mammals, due to their common possession of 

 these membranes, are known as the amniotes. The reptiles and birds 

 in addition to these membranes have shells for further protection against 

 the rigors of a terrestrial existence, while the higher mammals retain 

 the developing fetus in the uterus for even more effective protection. 



Yolk Sac. — A prominent yolk sac is present in the embryos of all 

 vertebrates having meroblastic cleavage. In them it represents a saclike 

 expansion of the ventral wall of the intestine overlaid with a layer of 

 splanchnic mesoderm. These two layers together make up the splan- 

 chnopleurc. The mesodermal portion is supplied with blood vessels which 

 deliver the absorbed yolk material to the embryo. The yolk sac be- 

 comes smaller as the yolk is utilized; in the chick, shortly before hatch- 

 ing, it slips into the belly cavity. 



In the higher mammals, the yolk sac is also present but only as a 

 small nonfunctioning relic. 



The Amnion and Chorion. — These membranes, which are actually 

 concentric sacs, arise as upward foldings of the body wall of the embryo. 

 These folds consist of ectoderm and somatic mesoderm, together known 

 as somatopleurc. In reptiles and birds, these sacs arise first as two folds, 

 with one upward folding appearing just in front of the embryo, the other 

 just behind. Gradually they grow upward and meet and fuse above the 

 embryo. This produces the double sac: the inner amnion surround- 

 ing the embryo and the outer chorion next to the eggshell. These 

 relationships are nearly identical with those of the mammalian embryo. 

 The amniotic sac is filled with amniotic fluid, and the embryo floats in 

 its private "pond." Clearly all terrestrial vertebrates are "aquatic" 

 at least in their embryonic stages. In this liquid medium, the develop- 

 ing embryo is free to maintain its shape and is effectively shielded from 

 mechanical injuries, 



