REPRODUCTION 53 



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 vigorously through a rich system of small vessels lying close 

 to the inner surface of the shell. The shell is porous. Thus ready inter- 

 change 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) immediately enclosing the yolk 

 mass is highly vascular and its vessels, like those of the allantois, com- 

 municate 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 Mammals. Primitive mammals, as indicated by such surviving 

 examples as Ornithorhynchus and Echidna, must have retained reptilian 

 methods of reproduction. However that may be, the two animals just 

 mentioned are oviparous. In size and possession of a protective shell, 

 the eggs resemble those of reptiles. 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 essentially as do reptiles. The material providing for develop- 

 ment and growth up to the time of hatching is all derived from the initial 

 yolk content of the egg. 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 i8i) developed in and by the abdominal skin serves to prolong the 

 period of dependence on the maternal food. The additional growth thus 

 made possible renders the young animal the more fit to succeed when it 

 finally starts out for itself. 



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 



