136 CELLS OF THE VERTEBRATE BODY 



Both nucleus and cytosome are necessary for the normal activ- 

 ities of the cell. It is not entirely possible to define the part each 

 plays in the metabolism of the whole. Cells that are deprived 

 of their nuclei are unable to carry on assimilation, although 

 dissimilation goes on until the cytoplasm is exhausted. This fact 

 and other types of experimental evidence would seem to indicate 

 that the nucleus may be a place where enzymes necessary for assimi- 

 lation are produced, but that the cytosome is the principal region of 

 synthetic activity and energy transformation in the cell. Whether 

 or not such a distinction can be sharply drawn, it cannot be 

 doubted that there is very close interdependence between these 

 two morphological divisions, and that the life of the cell depends 

 upon balanced interactions between nucleus and cytosome. 



Cell Division 



When cells were first discovered they were thought to arise spon- 

 taneously by a sort of crystallization. The nucleus was inter- 

 preted by some early investigators as a new cell in process of forma- 

 tion. As the microscope was perfected and more observations 

 were made, it was found that new cells were formed as the result of 

 the division of previously existing cells, and in no other manner. 

 It will be recalled that the amount of protoplasm increases when 

 assimilation exceeds dissimilation. During this period the cell is 

 said to be in the vegetative or nutritive stage. The cell is some- 

 times referred to, at this time, as a " resting " cell, but nothing 

 could less adequately describe it during this period of metabolic 

 activity. When the cell has reached a certain size it divides. 

 Whether or not cell size is the only factor governing cell division, 

 it is certainly a very important one. Cells divide by two different 

 methods: amitosis, or direct cell division; and initosis. or indirect 

 cell division. Mitosis is by far the more common method. 



Amitosis. — In amitosis or direct cell division, the nucleus of 

 the cell becomes somewhat elongated and is pinched into two parts 

 which are about equal in volume. The nuclei of certain types of 

 cells may divide amitotically without division of their cytosomes 

 and thus give rise to multinucleate cells. In complete amitosis, 

 after the nucleus is constricted the cytosome divides and two new 

 cells are formed. In this direct process of division the distribution 

 of cell components is only approximately equal. Such a type of 



