FROM EGG TO ADULT 



295 



FIG. 330. Strings of toads' eggs each of which is a single cell. Photograph nat- 

 ural size. 



being deposited in the water. An hour later four cells 

 are visible, then eight cells, after which division occurs 

 more rapidly (Fig. 332). 



Many animals retain their eggs in the body during 

 development. Such are certain flesh flies, plant lice, 

 some of the sharks, the copperhead, rattlesnake, and most 

 mammals. 



Development of Young. In numerous species, the 

 young does not continue its development uninterruptedly 

 but at certain periods rests and undergoes remarkable 



