52 COMPARATIVE ANATOMY 



eggs on land, usually in some natural hollow or crevice which will afford 

 protection. Crocodiles and alligators build rude nests of sticks or any 

 available loose materials and the female may guard the eggs. Most 

 reptiles leave the eggs to be incubated by the warmth of the sun. 



Many lizards and snakes are viviparous. The utmost protection 

 being afforded by viviparity, the importance of the protective shell is 

 reduced. 



A prolonged developmental period in course of which the young 

 animal attains large bulk — all internal organs, except the reproductive, 

 fully formed and highly differentiated before the animal hatches — the 

 heart beating vigorously and the blood circulating rapidly — all of this 

 going on within a thick shell — here is a set of circumstances which make 

 respiration a problem. In warm-blooded birds the maintenance of high 

 temperature during development accentuates the need of suitable provi- 

 sion for supplying abundant oxygen. The outstanding feature of the 

 development of the reptile or bird appears when the embryo itself goes 

 about the business of constructing a complex system of membranes so 

 disposed and so equipped with blood-vessels as to serve very efficiently 

 not only for respiration but for some other and secondary functions. 



Early in development, at a time when the main organs are in process 

 of formation (Fig. 84), the outer layer of the embryo, representing the 

 prospective body-wall of the animal, throws up a system of folds which 

 arch over and ultimately enclose the whole of the definitive embryo — much 

 as if an animal should enwrap itself in a highly exaggerated fold of its own 

 skin. Thus are formed the investing membranes known as the amnion 

 and the chorion. The amnion is derived from the inner layer of the fold, 

 the chorion from the outer. The amnion does not fit the embryo snugly. 

 The intervening space is occupied by a watery solution whose chemical 

 constitution resembles that of blood — and also resembles that of sea 

 water. Thus the embryo during its further development is bathed by a 

 fluid whose chemical nature is compatible with that of the embryonic 

 tissues. Further, immersion of the embryo in watery fluid affords the 

 best possible protection from externally caused mechanical pressures and 

 impacts. 



Meanwhile the enormous yolk mass has been enclosed (Figs. 84 

 and 87) by cellular layers which are prospectively the wall of the digestive 

 tube. Then from the hinder region of the embryonic digestive tube a sac 

 bulges out ventrally and, like a great and growing hernia, pushes beyond 

 the ventral body wall. Having thus attained the exterior of the embryo 

 proper, it becomes vastly expanded (by growth) and eventually becomes 

 spread out so that the greater part of its outer surface is, in conjunction 

 with the chorion, in close relation to the inner surface of the egg-shell. 

 This sac is the allantois. It becomes highly vascular, its arteries and 



