32 NORMAL HISTOLOGY. 



1. The cytoplasm, which usually makes up the chief bulk of the 

 cell, especially in those varieties which have active metabolic functions, 

 appears to be the part of the cell in which the assimilated food is utilized 

 in the production of chemical substances, either fresh cytoplasm or 

 some other product, or in the execution of movements or the libe- 

 ration of energy in other forms. Most of the active processes that 

 arc obvious seem to be carried on in the cytoplasm during the greater 

 part of the life-history of the cell. 



2. The nucleus appears to preside over the assimilative processes 

 within the cell. If a cell be subdivided so that the uninjured nu- 

 cleus is retained in one of the portions, that portion may grow and 

 become a perfect cell. But the portions that are deprived of a nu- 

 cleus do not grow, and while they may retain life for a considerable 

 time, utilizing the assimilated food they retain, eventually perish. 



Aside from this assimilative function, the nucleus appears to be 

 the carrier of hereditary characters from the parent cell to its prog- 

 eny during the division of the cell. This will become clearer when 

 the process of cell-division is described. 



3. The centrosome appears to be the organ presiding over the 

 division of the cell. It inaugurates those activities in nucleus and 

 cytoplasm which result in the production of new cells, and seems to 

 guide them, at least during the greater part of the whole process. 



It is evident, from these statements, that the cell has an exceed- 

 ingly complex organization, which a simple microscopical study can- 

 not wholly reveal. Notwithstanding this fact, obvious microscopical 

 differences are presented by cells which have become specialized in 

 different directions, and we must know something of the visible 

 structure of the primitive cell before we can appreciate these depart- 

 ures from it. 



The cytoplasm is not a simple substance. Its constitution is so com- 

 plex that our present means of research are not adequate to reveal 

 its structure. We know that its solid constituents are chiefly pro- 

 teids, together with relatively small quantities of carbohydrates, fats, 

 and salts. To these is added a large proportion of water which, 

 while not entering into a definite chemical union with the other 

 constituents, is so intimately associated with them as to form an 

 integral part of the cytoplasm. 



The visible structure of cytoplasm differs somewhat in different 

 cells, even among those that appear to be comparatively unspecial- 

 ized. In the fixed cells of the higher animals and man it appears 



