REPRODUCTION 7,7, 



called a ''permanent larva." Sexual maturity is ordinarily not attained 

 during the larval state. But the Mexican axolotyl, the larva of the sala- 

 mander, Ambystoma tigrinum, regularly breeds in the larval state. 



The diversity of reproductive arrangements in amphibians is in marked 

 contrast to the uniformity which prevails in reptiles and birds. The large 

 yolk-mass of the eggs of these animals makes possible a long develop- 

 mental period during which the young can attain relatively great size. A 

 newly hatched alligator is gigantic compared to a newly hatched salmon. 

 These circumstances, together with the fact that development takes 

 place within a thick shell, make necessary some special provision whereby 

 food derived from the yolk may be made accessible to all parts of the large 

 embryo and an adequate supply of oxygen obtained from some external 

 source. 



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 eflEiciently 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. 74), 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 (serosa). 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 develop- 

 ment 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 (Fig. 74) 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 (Fig. 74) 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 spreads 

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

 the chorion, in close relation to an extensive area of the inner surface 

 of the egg-shell. This sac is the allantois. It becomes highly vascular, 

 its arteries and veins communicating with the main vessels of the embryo. 



