CELLS. 



75 



Not uncommonly, the nucleus contains in its interior a 

 still more minute solid body or particle, which is known as 

 the " nucleolus." The functions, however, of the nucleolus 

 are not known with any precision, and it is often absent. 



CELL-MULTIPLICATION. When once formed, cells not 

 only grow and maintain their existence during an active 

 period of varying length, but they have also the power, in 

 many cases, of producing fresh cells by a process of cell- 

 multiplication or " cytogenesis." The modes in which this 

 is effected vary in different cases, but they may be reduced 

 to three principal forms : 



a. Endogenous Cell-multiplication. In this method new 

 cells are produced within a parent-cell by the separation of 

 the cell-contents into a greater or less number of distinct 

 masses, each of which may become ultimately enclosed in a 

 proper cell -wall (fig. 

 21). This method of 

 multiplication is well 

 seen in the fecundated 

 ovum, and it appears 

 to commence by the 

 cleavage or division 

 into two parts of the 

 nucleus. The cell - 

 contents then become 



aggregated round each half of the nucleus so as to form two 

 cells within the parent-cell. The nuclei divide again in a 

 similar manner, giving rise to four cells ; these divide again, 

 giving rise to eight cells ; and so the process may go on, 

 till there is formed in the primitive cell an indefinite num- 

 ber of new cells.* 



a b c 



Fig. 21. Cleavage of the yolk of the ovum of 

 Ascdris nigrovenosa (after Kolliker). 



* The term of "endogenous cell-multiplication" was originally applied 

 to cases in which the cell-contents divided into fresh cells without any 

 participation of the cell-wall. It is now known, however, that the cell- 

 wall never takes any part in the process of cell-multiplication. It has 

 been proposed, therefore, to restrict the term "endogenous" to that 



