DEVELOPMENT OF OTHER VERTEBRATES 



423 



debris (Fig. 221), and the female apparently remains in the vicinity 

 until the young hatch as juvenile alligators some five or six inches 

 in length. 



Most snakes and lizards lay eggs in a similar manner, con- 

 cealing them in various ways (Fig. 222). Some snakes and lizards 

 are ovoviviparous, however, since the embryo develops from a 

 large yolk-laden egg which is retained within the body of the 



Fig. 220.— Development of turtle. A, embryo drawn on a .arge scale show- 

 ing gill slits. B, stage comparable with F, but showing yolk sac. C, later 

 embryo drawn on same scale as A. DtoF, stages within egg. G, hatching, 



H, just after hatching. 

 (After Agassiz, redrawn by George T. Kline.) 



female. Neither of these cases resembles the mode of devel- 

 opment in mammals, which will be described in a later section 

 of the present chapter, since the embryo is nourished primarily 

 by the yolk and not by a diffusion of food from the blood of the 

 parent to that of the embryo. The body of the parent merely 

 functions as an incubator for the eggs of such a fish or reptile, 

 whereas the relationship between the mammal and its offspring 

 is more hke that of host and parasite. 



