136 EMBRYOLOGY. 



outside, (Fig. 77, o,) or inside, at the bottom of the cavity 

 of the body. Some insects, such as the gad-flies, deposit 

 their eggs on other animals. Finally, many abandon their 

 eggs to the elements, taking no further care of them after 

 they have been laid ; such is the case with most fishes, some 

 insects, and many mollusks. As a general rule, it may be 

 said that animals take the more care of their eggs and brood 

 as they occupy a higher rank in their respective classes. 



284. The development of the embryo does not always 

 take place immediately after the egg is laid. A considera- 

 ble time, even, may elapse before it commences. Thus, the 

 first 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 most butterflies, and of insects in general, are 

 laid in autumn, in temperate climates, and remain unchanged 

 until the following spring. During 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 manner in plants. The seeds, which are 

 equivalent to eggs, preserve for years, and even for ages, 

 their power to germinate. Thus, there are some well- 

 authenticated cases in which wheat taken from the ancient 

 catacombs of Egypt has been made to sprout and grow. 



285. A certain degree of warmth is requisite for the 

 hatching of eggs. Those of birds, especially, require to be 

 submitted, for a certain length of time, to a uniform tem- 

 perature, corresponding to the natural heat of the future 

 chicken, which is naturally supplied by the body of the 

 parent. In other words, incubation is necessary for their 

 growth. Incubation, however, is not a purely vital phenom- 

 enon, but may be easily imitated artificially. 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, leaving them to be hatched by the sun. In 



