\6o THE INFANCY OF ANIMALS 



cavity is made some five inches deep. Here the eggs are 

 laid, and distributed over the bottom by the feet, when the 

 soil is thrust back again and beaten down flat : finally, 

 the spot is concealed by scratching the surface a little with 

 the claws. But this done, no further interest is displayed 

 in the welfare of the nest. 



While in the majority of cases the eggs of reptiles are 

 laid as soon as the shell has been formed around them, in 

 some species they are retained within the body until the 

 development of the embryo is complete. What determines 

 which of the two courses shall prevail, is not clear, but it 

 would seem that internal development must be regarded 

 as a device to secure the safety of the young. In the sea- 

 snakes, which are ovo-viviparous, the advantage is obvious, 

 since eggs deposited in the water would be liable to be 

 carried away, or devoured by predaceous animals. But 

 it is not so easy to understand why the slow-worm, burrow- 

 ing-snakes, the viper, and the skinks, for example, should 

 agree with the sea-snakes in this particular. A closer 

 study of the habits of these reptiles in a wild state may 

 clear up the mystery. It is suggestive, at any rate, to 

 find degrees of ovo-viviparity tending to develop into true 

 viviparity — that is to say, the condition where the young, 

 instead of being nourished by a mass of food-yolk enclosed 

 within a shell, are nurtured by absorption from the tissues 

 of the parent. In some of the skinks there seems to be 

 an approach towards this condition, the egg having no 

 hard shell, and containing but a limited amount of food- 

 yolk, which is replaced by the absorption of food through 

 the thin membrane which constitutes the inner lining of 

 the shell in normal eggs. 



Whether the eggs be laid as soon as the shell is com- 

 pleted, or whether they be retained within the body, the 



