44 GENERAL EMBRYOLOGY 



usually associated with special forms of cell activity. Conditions 

 in some of the Protozoa suggest that primitively division of the 

 nucleus or the multiplication of the nuclear bodies might not 

 have been associated with a corresponding division of the 

 cytoplasmic body, but that these originally independent 

 divisions have gradually come to be uniformly associated. 



It has commonly been supposed that the direct form of cell 

 division occurs but rarely and then usually in cells which are 

 moribund. It is becoming clear, however, that amitosis is in 

 reality not particularly infrequent. It seems to occur normally 

 in many tissues (mesenchyme), and often where there is an un- 

 usual or a sudden increase in nuclear activity and energy 

 expenditure on the part of the whole cell, as in ovarian follicle 

 cells and tapetal cells, muscle cells, rapidly growing or rediffer- 

 entiating cells in regenerating tissues ; it is also true that amitosis 

 is frequent in such tissues as stratified epithelia whose cells 

 are nearing the end of their life or activity. 



The second and more usual method of cell fission is that 

 termed indirect cell division, or mitosis (Flemming), or karyo- 

 kinesis (Schleicher). This is a complicated process involving 

 the establishment and operation of an intricate mechanism 

 within the cell, shared in by nearly all its living parts. The 

 essential result of the division of the cell by the action of this 

 complex mechanism concerns in particular the chromatic sub- 

 stance of the nucleus, for in nearly all known instances the 

 chromatin sharing in the process is very precisely divided into 

 two equal portions, each of which goes to one of the daughter 

 cells. We may give here only a brief outline of the essentials of 

 this process of mitosis, again describing an imaginary schema 

 with which to compare later some of the variations in detail 

 shown by actual cells. 



As the first step in mitosis we should consider the division of 

 the centrosome into two, which remain lying together within 

 the undivided kinoplasmic centrosphere. This division of the 

 centrosome is usually quite removed in point of time from the 

 other phenomena of mitosis, for it occurs normally during the 

 reconstruction of a daughter cell immediately after its formation, 



