CHAPTEK XVI. 



THE SHAPES OF ANIMAL CELtS. 



§ 260. Among animals as among plants, the laws of mor- 

 phological differentiation must be conformed to by the mor- 

 phological units, as well as by the larger parts and the wholes 

 formed of them. It remains here to point out that the con- 

 formity is traceable where the conditions are simple. 



In the shapes assumed by those rapidly-multiplying cells 

 out of which each animal is developed, there is a conspicuous 

 subordination to the surrounding actions. 

 Fig. 294 represents the cellular embryonic 

 mass that arises by repeated spontaneous 

 fissions. In it we see how the cells, origin- 

 ally spherical, are changed by pressure 

 against one another and against the limit- 

 ing membrane ; and how their likenesses 

 and unHkenesses are determined by the likenesses and un- 

 likenesses of the forces to which they are exposed. This fact 

 may be thought scarcely worth pointing out. But it is 

 worth pointing out, because what is here so obvious a con- 

 sequence of mechanical actions, is in other cases a conse- 

 quence of actions composite in their kinds and involved in 

 their distribution. Just as the equalities and inequalities of 

 dimensions among aggregated cells, are here caused by the 

 equaHties and inequalities among their mutual pressures in 

 different directions ; so, though less manifestly, the equalities 



