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 
Fic. 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 
asinchemical composition. It is by no means homogeneous; 
adjacent parts are different in properties and aptitudes. The 
nucleus, which is more readily seen than other cell elements, 
