392 ZOOLOGY 



lizards and snakes retain the eggs in the oviduct until the em- 

 bryo is hatched, thus bringing forth their young alive. Many 

 forms deposit their eggs in the warm sand or earth or in decay- 

 ing rubbish heaps, where the abundant heat is favorable for the 

 developing young. 



Much yolk is present in the egg and segmentation is partial, 

 being confined to a disc. The germinal layers and the im- 

 portant organs develop about the axis of this disc, the outer 

 margins of which spread over the whole yolk in the form of a 

 sac designed to nourish the embryo. The details of the growth 

 are entirely too complicated for statement here. Two important 

 embryonic membranes — the amnion and allantois — appear for 

 the first time (see also §440). The amnion consists of folds 

 of the blastodermic disc which arise, surrounding the embryo 

 at its margin. These folds grow dorsally over the embryo and 

 ultimately fuse to enclose a space which becomes filled with 

 fluid. The amnion folds include both ectoderm and mesoderm. 

 It is protective in function (Fig. 208, am). The cavity between 

 the two layers of the amnion is an outgrowth of the coelom. 

 The allantois arises as an evagination from the posterior portion 

 of the digestive tract, and is made up of entoderm and meso- 

 derm. It grows into the cavity of the amnion. It finally sur- 

 rounds not merely the embryo but the yolk on the ventral side, 

 and being well supplied with blood vessels is most important in 

 supplying the embryo with oxygen. In this and in other 

 features the reptiles show a close kinship with the birds. 



424. Place in Nature. — Reptiles live upon both plant and 

 animal food. They devour worms, insects, fish, amphibians, 

 birds, small mammals, and eggs. Not many animals depend 

 on reptiles for food. They are, perhaps, more immune from 

 attack than most animals. Some of the predaceous birds eat 

 lizards and snakes. 



The group does not aid man at many points. Certain large 

 lizards, and the tortoises and turtles furnish food. The soft- 

 shelled turtle, the green-turtle, and the diamond-back terrapin 

 are the most prized. 



Skins of crocodiles and of some snakes, and the horny tor- 



