CHAPTER VI 



The multiplication of cells Mitotic and amitotic nuclear division. 



WE have already seen that the possibilities of structural 

 differentiation within the limits of the individual cell are by no 

 means exhausted by the distinction between cytoplasm and 

 nucleus, but it is only when we come to study in detail the pro- 

 cess of cell- division that we begin to gain any adequate concep- 

 tion of the fundamental complexity of the organic unit. We 

 have hitherto spoken of this process, as it occurs for example in 

 Amoeba, as though it were a simple matter, initiated by constric- 

 tion of the nucleus into two parts and concluded by a correspond- 

 ing division of the cytoplasm. The researches of the last forty 

 years, however, rendered possible by the improvements in 

 microscopical apparatus and micro-chemical technique, have 

 taught us that in the vast majority of cases the process of cell- 

 division is one of extreme complexity, accompanied by remark- 

 able phenomena which reveal a"previously unsuspected degree of 

 structural differentiation within the nucleus itself. To these 

 phenomena Schleicher in 1878 gave the name karyokinesis, for 

 which Flemming, in 1882, proposed to substitute mitosis. Both 

 these terms are still in common use. 



In a typical cell (Fig. 30) the cytoplasm (cyt.) is a semi-liquid 

 substance usually enclosed in a thin cell-membrane (animal cells) 

 or a thicker cell- wall (plant cells). It exhibits a microscopic 

 structure which is variously interpreted as reticular (fibrillar), 

 alveolar (foam-like), or simply granular, the probability being that 

 the real truth is expressed by a combination of these different 

 views. It may or may not contain plastids of various kinds (e.g. 

 chloroplastids in green plants). 



The nucleus (nu.\ consisting of the so-called nucleoplasm or 

 karyoplasm, is usually a spherical, more or less centrally situated 

 body enclosed in a definite nuclear membrane (n.m.). Within 

 this membrane the karyoplasm is differentiated into various con- 

 stituents. In the first place there is a network or reticulum of 



