REPRODUCTION 



533 



Fig. 21-3. Some snakes, such as the green snake shown here, lay eggs. The outer shell is leathery and wrinkled. 



tiny bit of its ancestral aquatic world had to 

 be incorporated into the egg. This was sup- 

 plied by the introduction of the amnion ( p. 

 320). Moreover, as the embryo advanced in 

 its development it required more oxygen 

 than could be supplied by diffusion to the 

 embryo directly. The development of the 

 allantois (p. 320) met this need (Fig. 21-4). 

 Large eggs, together with their extraem- 

 bryonic membranes, served very well for 

 the reptiles and birds. The former reached 

 great heights as dominant worldwide ani- 

 mals during the Mesozoic Era and the latter 

 are a dominant form today. 



However, the keen swift-moving mam- 

 mals that were to follow developed a new 

 approach to this problem. When their em- 

 bryo began to receive nourishment from 

 the uterine wall via a special organ, the 

 placenta (p. 338), the large food reserve of 

 the reptilian egg was no longer necessary, 

 although the membranes of the latter were 

 retained. Gradually the yolk disappeared. 



but the yolk sac, though empty, still re- 

 mained (Fig. 21-4). Let us examine these 

 extraembryonic membranes a little more 

 carefully. 



The amnion is an outfolding of the body 

 wall of the embryo and is lined with peri- 

 toneum, the lining of the coelomic cavity. 

 This membrane continues to grow around 

 tlie embryo until the latter is completely 

 enveloped and lies in the resulting amni- 

 onic cavity. The membranes fuse at their 

 point of juncture, so that the cavity is a 

 closed, fluid-filled sac reminiscent of the 

 aquatic environment that was the home of 

 all earlier embryos. Because the amnion is 

 double-walled, an outer layer, called the 

 chorion, is formed. In birds and reptiles the 

 chorion comes in contact with the inner 

 layer of the egg shell, whereas in mammals 

 it comes in contact with the uterine wall 

 and ultimately becomes a part of the pla- 

 centa. 



The allantois arises as an outpushing from 



