34 CHORDATE ANATOMY 



A considerable part of the blood of the embryo is diverted into the allantoic 

 arteries and circulates vigorously 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 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. 74) immediately enclosing 

 the yolk-mass is highly vascular and its 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 

 specialization and efi&ciency in birds. Correlated with the greatly 

 increased protection afforded during development, relatively few eggs 

 are produced. 



Primitive mammals, as indicated by such surviving examples as 

 Ornithorhynchus and Echidna, must have retained reptilian methods of 

 reproduction. The duck-bill, a burrowing animal, deposits the eggs 

 (usually two) in the burrow. Echidna, producing usually only one egg 

 in a season, places the egg in a fold of abdominal skin, a temporary 

 marsupium, where it is carried and incubated by the warmth of the body 

 until the young hatches. The embryos of these two mammals develop 

 amnion, chorion, allantois, and allantoic and yolk-sac circulations essen- 

 tially as do reptiles. The one new thing which these animals do is to 

 provide the young with a convenient source of food to serve for a time 

 immediately after hatching. Milk produced by mammary glands (see 

 page 127) developed in and by the abdominal skin serves to prolong the 

 period of dependence on the maternal food. 



All known existing mammals except the duck-bill and spiny ant-eater 

 are viviparous. The minute eggs contain so little yolk that they could 

 never pass beyond the very early stages of development unless additional 

 food material were somehow provided. In the great majority of mammals 

 this is done by means of an organ which is one of the most characteristic 

 features of a mammal. The egg, liberated from the ovary and fertilized, 

 becomes caught and lodged in the superficial tissue of the uterine wall. 



