96 SEX-LORti 



nourishment during the fast, is gradually absorbed, 

 and becomes a mere stump. The tadpole has now 

 become a frog and leaves the water to live on land. 



Coming to the reptiles, we find that here the embryo 

 develops completely within the hard shell of the egg, 

 which also contains the yolk for its sustenance. The 

 female lays the fertilized eggs in some hole which she 

 has dug. The eggs are hatched out when the young 

 have attained the mature stage. 



Of course, it is well known that birds are hatched 

 from eggs (Fig. 36). As in the reptiles, the eggs are 

 protected by a hard shell containing, besides the 

 embryo, the yolk, which is by far the greater in bulk. 

 The chicks when fully developed crack the shell with 

 their beaks and emerge. In a very short time they 

 are capable of looking after themselves. 



A most interesting transition stage is to be found 

 between animals that lay eggs and those that give 

 birth to living young. Thus we find among fishes 

 and also among reptiles some species whose eggs 

 are hatched inside the mother's body, after being 

 fertilized by th,e male. For instance, the shark and 

 dogfish give birth to living young, these being passed 

 out one at a time either every day or every second 

 day for a month or two during the warm season. 

 On the other hand, we find that the lowest mammals 

 still lay eggs, which are hatched outside the body. 



Mammals are animals which have breasts (mammce) ; 

 these contain milk glands, which produce milk for the 

 nourishment of the young. The duck-billed platypus 

 and the spiny ant-eater (echidna) both lay eggs. The 

 former lays her eggs in a nest made in a burrow and 

 broods over them like a bird; but she feeds the newly 



