CHAPTER III 



Cell Division 



WE have seen that when a relatively undifferentiated 

 cell grows to a certain size it divides, possibly in 

 consequence of a disturbance in the normal volume-ratio 

 between nucleus and cytoplasm. The division of the cell is 

 always preceded by division of the nucleus, and this may 

 take place in two ways. By far the commoner method of 

 nuclear division is the rather complicated process known as 

 karyokinesis, or mitosis, but in many of the Protozoa and 

 occasionally in the Metazoa direct or amitotic division of the 

 nucleus occurs. It will be convenient to consider amitotic 

 division first. 



In a cell which divides amitotically the nucleus elongates, 

 becomes constricted in the middle so as to assume a some- 

 what dumb-bell-shaped form, and then the two halves 

 separate and move apart from each other, after which the 

 cytoplasm becomes similarly constricted and divides between 

 the two nuclei, giving rise to two daughter cells (PL I). In a 

 Protozoon the whole series of events can be watched in the 

 living organism, and no doubt can arise with regard to the 

 successive stages of the process. A closely similar series of 

 stages has frequently been described in sections of the 

 tissues of various Metazoa (e.g. the tapeworm, Moniezia 

 (CHILD), various insects, the epididymis of the mouse 

 (JORDAN)) and there can be no doubt that in some cases, 

 and possibly quite frequently, Metazoan cells divide in this 

 manner. It is sometimes said that in the higher animals 



