REPTILIA 391 



a period of incubation which usually occurs outside the body; 

 though some lizards and snakes retain the eggs in a special por- 

 tion of the oviduct until the embryo is hatched, thus bringing 

 forth their young alive. Many forms deposit their eggs in the 

 warm sand or earth or in decaying 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 blast odermic 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 ccelom. 

 The allantois arises as a fold from the posterior portion of the 

 digestive tract, and is made up of entoderm and mesoderm. 

 It finally surrounds 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 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- 



