CHAPTERS 

 CELL DIVISION 



When cells .were first discovered, and even after it became fairly cer- 

 tain that all organisms were composed of them, no one appreciated how 

 fundamentally the cells Avere involved in the constitution of living things. 

 They were thought, for example, to be of secondary origin; that is, 

 animals and plants were believed to possess a formative or nutritive 

 substance without any particular organization or structure, and out 

 of this the cells were supposed to be formed. While all organisms were 

 found to contain cells, it was not thought that these cells had any neces- 

 sary function in the production of new cells out of the formative material. 

 Gradually, however, the idea gained ground that the origin of new cells 

 occurred by division of old cells, a doctrine which in 1855 was expressed 

 by the famous pathologist Virchow in the words omnis cellula e cellula — 

 all cells from cells. While the origin of cells from cells was thus early 

 recognized, the mechanism by which cells originated from other cells 

 was not known until twenty or thirty years later. It was not until 

 1873 that the common method of cell division — resolution of the chroma- 

 tin into distinct separate bodies and the formation of a spindlelike 

 mechanism manipulating these bodies — was discovered. The same 

 method was soon witnessed in a variety of plants and animals and is 

 now found to be nearly universal. To this method of cell division the 

 names mitosis and karyokinesis are applied. The latter is the more 

 descriptive, but the former is more often used. 



Interphase. — A cell not in division is said to be in interphase. In 

 such a cell the chromatin is so diffuse as to present the appearance of a 

 network (Fig. 35 A). Actually, in most cells, this chromatin exists in 

 a number of distinct portions, the chromosomes; but the threadlike form 

 which these chromosomes take in most animals makes it impossible to 

 distinguish them. In a few organisms (some grasses among them) the 

 chromosomes are more condensed and are separately visible even in 

 the interphase. In some special tissues, such as the salivarj^ glands of 

 flies, the chromosomes are greatly enlarged and are more easily recog- 

 nizable in interphase than in any cell division. The chromosomes of 

 these glands also have a pattern by which they can be distinguished; 

 and every nucleus has a set of chromosomes identical in pattern with 

 those of any other nucleus. The individuality of the chromosomes which 



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