HATCHING OF EGGS. 101 



course the more weighty part always descends, in 

 every position of the egg, as far as they uill permit, 

 and the chick being thence prevented from sliding 

 down, nourishes itself in security. 



We cannot, on account of their minuteness, ascer- 

 tain whether there is any similar mechanical contriv- 

 ance in the eggs of insects ; but we have in several 

 instances distinctly observed the speck where the 

 embryo insect was placed just within the shell of 

 the egg. In order to stimulate it to feast and 

 fatten on the good things stored up in his egg-shell 

 chamber, it appears that a certain degree of heat 

 is indispensably requisite ; for cold, though it does 

 not usually, as we have seen, kill the embryo, almost 

 always renders it torpid. But the stimulus of heat 

 produces activity in the living principle, causes the 

 embryo to devour all the nutritive contents of the 

 egg, and thence to increase proportionably in size. 

 It is worthy of remark, however, that the stimulus of 

 light, contrary to that of heat, acts unfavourably upon 

 the hatching of eggs. Both of these positions may 

 be illustrated by numerous facts and experiments. 



Most birds, so far as has been ascertained, supply 

 the heat necessary for hatching their eggs by sitting 

 constantly upon them during a certain number of 

 days ; but reptiles, such as the crocodile, bury their 

 eggs in the warm sand upon the banks of rivers. 

 I Insects, again, seldom, if ever, sit upon their eggs, as 

 birds do, in order to hatch them. This, indeed, 

 would be impossible, as the greater number of insects 

 die in a few days after depositing their eggs, the con- 

 tinuation of the species being apparently their only 

 business in their last or perfect stage; since, as they 

 Ithen generally cease to feed, they cannot possibly live 

 i long. A few instances, however, have been observed, 

 of insects performing something very similar to the 



g3 



