THE DESCENT OF THE PRIMATES 35 



deniably a protective apparatus, which is com- 

 paratively more spacious in the younger and 

 more delicate stages than in the older embryos. 



Such a protective water-sac is evidently of 

 more paramount significance to an embryo that 

 resides inside its mother's generative organs — 

 where it is exposed to various pressures, peri- 

 staltic and otherwise — than to one which is 

 already protected either by a thick layer of 

 albumen or by a hard shell, or by both. And 

 so it seems more reasonable to look for its very 

 earliest origin rather among viviparous than 

 among oviparous animals. Take, for example, 

 the shark's and ray's eggs, with their black, 

 horny egg-case, the fluid albuminous contents, 

 and the yolk, on the top of which the young 

 undergoes its successive developmental changes. 

 We observe a close resemblance with a saurop- 

 sidan egg, the shell of which is not always hard 

 and calcareous, but in many cases of leathery 

 consistency (snakes, crocodiles). Mechanically 

 speaking, the eggs of both these classes of ver- 

 tebrates are similarly conditioned, and still 

 the sharks and rays never possess any amnion, 

 whereas the crocodiles, turtles, snakes, and birds 

 do. We may evidently not seek the starting- 

 point for the amnion formation in any peculiar 



