SEX AND REPRODUCTION 71 



matter, or, in gardens, manure-heaps, in which to deposit its 

 clusters of eggs. 



Turtles lay their eggs in holes scraped out in the sand of 

 tropical coasts well above high-water mark to the number of a 

 hundred or more; the holes being carefully filled up with sand 

 and the surface smoothed down. The period of incubation, by 

 the aid of the sun's heat, is believed to be at least seven weeks. 

 Turtles' eggs have a parchment-like covering, unlike the hard 

 calcareous shell of those of the giant land-tortoises. 



Crocodiles likewise generally lay their eggs in sand, and 

 leave them to be hatched by the sun's heat, although, in some 

 cases at any rate, they are visited from time to time by the 

 female parent, who assists the young ones in escaping from 

 their prisons. In the case of the Nile crocodile {Crocodilus ni- 

 loticus) in Madagascar, the eggs are laid in a pit in the sand 

 from one and a half to two feet in depth ; the centre of the pit 

 being rather higher than the margin, which is undermined, so 

 that the eggs naturally roll into the shelter thus formed. 

 The eggs are laid somewhat before daybreak ; and after half 

 the batch has been deposited, a layer of sand is spread for the 

 reception of the moiety. When the laying is completed the 

 pit is filled up with sand and the surface smoothed down. 

 The position of the nest is, however, frequently betrayed by 

 the mother taking up her position upon it at night. When the 

 young are hatched (which occurs in about twelve weeks after 

 the laying of the eggs), the nest is generally pulled to pieces, 

 and the empty egg-shells left lying scattered about. This 

 digging out of the nest is believed to be accomplished by the 

 mother, who appears to be warned when the eggs are ready to 

 be hatched by the piping cries uttered by the young crocodiles 

 while still in the shell. Apparently the eggs are dug out by the 

 parent several days before they actually hatch. The process 

 of breaking the shell is accomplished by the young crocodile 

 with the aid of the " egg-tooth " ; a two-cusped excrescence 

 on the summit of the upper jaw at the extremity of the muzzle ; 

 this acts in drill-fashion, and does not disappear till some weeks 

 after birth. Many other reptiles have a somewhat similar 

 weapon . 



In tropical America the female of the black caiman 

 [Caiman niger) covers up her eggs with a mass of bushes, and 



