COMPARED WITH THOSE OF ANIMALS. 57 



The multiplication of cells by division is common 

 among the infusoria, which increase by what is called the 

 fissiparous method of reproduction, in a manner precisely 

 analogous to the mode of increase among the parallel group 

 of plants. The parenchyma of the cell at first becomes 

 more opaque, a clear line is seen to form across its cavity, 

 and a sort of hour-glass contraction takes place along this 

 line. Each division now struggles to separate from its 

 fellow, and the separation is no sooner effected than the 

 two cells dart off in opposite directions, and rapidly assume 

 their normal size and figure. This division sometimes 

 takes place vertically, as in vorticella, and sometimes trans- 

 versely. In some of the infusoria, the Paramecia, for in- 

 stance, it occurs as often as three or four times a day. 



The multiplication of cells by division, is beautifully ap- 

 parent in the development of cells within the mammalian 

 ovum, which has especially a plant-like mode of growth. 

 The ova of all the different orders of the vertebrata, birds, 

 fishes, reptiles, as well as mammalia, have in the beginning 

 nearly the same uniform structure; and although it is by 

 differences in the processes of cell-multiplication whilst in 

 the embryonic condition, that those differences which cha- 

 racterize the full-grown animals are brought about, yet the 

 principle of division is in all ova precisely the same. The 

 process has been observed in the impregnated ovum of 

 Ascaris acuminata, one of the oviparous Entozoa, which is 

 a very favorable subject for the study of it, owing to the 

 transparency of its body, and it is found that the embryonic 

 mass commences very much after the same plan as in plants. 

 Fig. 14 shows the successive stages of segmentation. The 

 ovum having been impregnated, the yolk-bag slightly sepa- 

 rates from the enveloping membrane and subdivides into 

 two halves, each of these again subdivides into two more, 



