HATCHING OF EGGS. 101 



yolk, of course the mere weighty part always de- 

 scends, in every yiosition of the egg, as far as they 

 will permit, and the chick being thence prevented 

 from shding 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 em- 

 bryo 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 indispensa- 

 bly 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 in- 

 crease proportionably in size. It is worthy of reniark, 

 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 nume- 

 rous 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. 

 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 

 then generally cease to feed, they cannot possibly live 

 long. A few instances, however, have been observ- 

 ed, of insects performing something very similar to the 

 9# 



