Reproduction 241 



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 thai 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 (Fig. 239) 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. 239) and, like a great and growing hernia, 

 pushes beyond the ventral body-wall. Having thus attained the ex- 

 terior 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 eggshell. This sac is the allantois. It 

 becomes highly vascular, its arteries and veins communicating with 

 the main vessels of the embryo. A considerable part of the blood of 

 the embryo is diverted into the allantoic arteries and circulates vigor- 

 ously through a rich system of small vessels lying close to the inner 

 surface of the shell. The shell is porous. Thus ready interchange of 

 respiratory gases between the blood and the external air is provided 

 for. The allantoic sac serves also as a receptacle for embryonic waste. 

 The ducts from the embryonic kidneys open into the extreme hind end 

 of the digestive tube, whence the fluid excreted by the kidneys readily 

 passes into the cavity of the allantois. 



The inner cellular layer (yolk-sac: Fig. 239) immediately enclosing 

 the yolk-mass is highly vascular, and its blood-vessels, like those of 

 the allantois, communicate with the main arteries and veins of the 

 embryo. The blood circulating through these vitelline vessels picks up 

 dissolved yolk materials which are conveyed to all parts of the embryo, 

 thus making the yolk available everywhere for metabolism and growth. 



In viviparous reptiles, the amnion, the allantois with its vascular 

 system, and the yolk-sac circulation are developed as in the embryos of 

 oviparous reptiles. The oxygen obtained by the allantoic vessels, how- 

 ever, must be derived from the maternal blood in the wall of the 

 oviduct. 



In reptiles and birds, building of nests and parental care of young 

 are much more prevalent than in fishes and amphibians, reaching high 



