OF THE EGG. 107 



eggs laid by the hen do not begin to develop until the whole 

 number which is to constitute the brood is deposited. The 

 eggs of the butterfly and of most insects are laid in autumn, 

 and remain in the same condition until the following spring. 

 Daring this time the principle of life in the egg is not 

 extinct, but is simply inactive, or in a latent state. This 

 tenacity of life is displayed in a still more striking man- 

 ner in plants. The seeds, which are equivalent to eggs, 

 preserve for years, and even for ages, their power to germi- 

 nate. Thus, wheat taken from the catacombs of Egypt 

 has been made to sprout and grow in some well-authenti- 

 cated cases. 



285. A certain degree of warmth is requisite for the 

 hatching of eggs. Those of birds, especially, demand a uni- 

 form temperature, corresponding to the natural heat of the 

 future bird, to be constantly applied for a certain length of 

 time ; this is naturally supplied by the body of the parent. 

 In other words, incubation is necessary for their growth. 

 Incubation is not a purely vital phenomenon, but may be 

 readily imitated by artificial means. Some birds of warm 

 climates dispense with this task ; for example, the ostrich 

 often contents herself with depositing her eggs in the sand 

 of the desert, where they are hatched of themselves. In 

 like manner, the eggs of most birds may be hatched at will, 

 by maintaining them at the proper temperature. Before en- 

 tering into the details of embryonic transformations, a few 

 words are necessary respecting the composition of the egg. 



286. Composition of the Egg. The egg is composed 

 of several substances, varying in structure, as well as in 

 appearance. Thus, in a a new laid hen's egg (Fig. 101), 

 we have first a calcareous shell ; then an albuminous sub- 

 stance, the wldie ; within this the yolk ; and before it was 

 laid, there was in the midst of the latter a minute vesicle, 

 the germinative vesicle (Fig. 99, ^), containing a still smaller 



