THE CELL THEORY 



253 



groups are set apart to perform particular duties. The divi- 

 sion of physiological labor which arises at this time marks 

 the beginning of separate tissues. It has been demonstrated 

 over and over that all tissues are composed of cells and cell- 

 products, though in some instances they are much modified. 

 The living cells can be seen even in bone and cartilage, in 



Fig. 76. — The Egg and Early Stages in its Development. 



(After Gegenbaur.) 



which they are separated by a lifeless matrix, the latter being 

 the product of cellular activity. 



Fig. 77 shows a stage in the development of one of the 

 mollusks just as the differentiation of cells has commenced. 



The Nucleus. — To the earlier observers the protoplasm 

 appeared to be a structureless, jelly-like mass containing 

 granules and vacuoles; but closer acquaintance with it has 

 shown that it is in reality very complex in structure as well 

 as in chemical composition. It is by no means homogeneous : 

 adjacent parts are different in properties and aptitudes. The 

 nucleus, which is more readilv seen than other cell elements, 



