REPTILIA. 387 



Describe and attempt to explain the motion of snakes from actual 

 observation: in water; on land. 



. 415. Reproduction and Development. The ova escape 

 from the two ovaries into the body cavity. As in the Amphibia 

 the inner end of the oviduct opens well forward in the body- 

 cavity. The ova enter the oviducts, and during the descent are 

 fertilized. After fertilization the glands in the walls of the 

 oviducts add albumen and shell structures, as in the birds. 

 The eggs require a period of incubation which usually occurs 

 outside the body, though some lizards and snakes retain the 

 eggs in a special portion of the oviduct until the embryo is 

 hatched. 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 431). 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. 205, 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. 



