CHAPTER VIII 

 SOMATIC CELL-DIVISION 



In most organisms growth involves a multiplication of cells by divi- 

 sion. In unicellular forms this results in the multiplication of the 

 organisms themselves. In multicellular forms the protoplast which is 

 to undergo development into a new individual initiates a series of sub- 

 divisions which will eventuate in the many cells of the body, or soma. 

 Ordinarily each subdivision involves both the division of the nucleus 

 by a complicated process known as mitosis, or karyokinesis, and the 

 division of the cytosome, called cytokinesis. These two processes may 

 be correlated in various ways. 



In the present chapter typical somatic cell-division will be described 

 in its main outlines by way of preparation for detailed discussions of the 

 more specific and problematic points in subsequent chapters.^ 



Outline of Somatic Mitosis. — The remarkable character of the process 

 of mitosis finds its meaning in the peculiar organization of the nucleus. 

 The nucleus contains a number of well-individualized units, the chromo- 

 somes, each of which in turn has a characteristic organization. In 

 normal cell-division and differentiation it appears to be essential that 

 each chromosome should be so divided that this organization, and hence 

 the organization of the nucleus as a whole, will be reproduced in each 

 of the two daughter nuclei. 



In such a tissue as that of the root tip the nucleus in the metabolic 

 condition consists of a bounding membrane, a mass of karyolymph, a 

 reticulum composed chiefly of thread-like chromosomal elements (chrom- 

 onemata or their persistent basis), and one or more nucleoli. As the 

 prophase of mitosis begins (Fig. 54), the reticulum gradually separates 

 into its constituent chromonemata through the disappearance of the 

 small strands connecting them. At first the chromonemata are thin 

 and crooked. They are also longitudinally double; in some cases this 

 split condition is visible from the very beginning of the prophase. 



The double chromonemata soon become straighter and thicker. As 

 they do so their split condition becomes more obvious, so that they 

 appear as double and somewhat twisted threads imbedded in the karyo- 

 lymph. Their arrangement is often very irregular, but in rapidly multi- 



' A useful list of works on mitosis in angiosperms is given by Picard (1913). Ruys 

 (1925) lists the angiosperm genera in which nuclear studies have been made. A 

 valuable review of researches on many chromosome problems is given by Reuter 

 (1930). See also the works of Tischler (1921-1922) and Schiirhoff (1926). 



105 



