SOMATIC CELL-DIVISION 113 



the thickened double chromosomes are arranged with their attachment 

 regions in the equator of the spindle, which is formed chiefly by the 

 karyolymph. In the anaphase the longitudinal halves of each double 

 chromosome pass toward opposite poles of the spindle. In the telophase 

 the chromosomes of the two resulting groups reorganize two daughter 

 nuclei, the chromonemata becoming joined to form the reticula while the 

 matrix substance becomes less evident. 



Anticipating evidence to be presented in following chapters, it may be 

 stated further that in any particular kind of organism the chromosomes in 

 the nucleus occur in a characteristic number and are differentiated among 

 themselves in function and frequently in visible structure; they con- 

 stitute a definitely organized system. Moreover, this organization is 

 maintained by virtue of the genetic continuity of essential constituents 

 of each chromosome through successive nuclear generations. This means 

 that the chromosomes are not merely bodies w^hich are temporarily formed 

 by the nucleus but represent definite and persistent individuals repro- 

 ducing by division and passing through a complicated series of visible 

 changes in each nuclear cycle. 



In view of these facts, it appears that the significant feature of somatic 

 mitosis is this: each chromosome is longitudinally divided into equal halves 

 ivhich are distributed to the two daughter nuclei. These two nuclei are 

 consequently similar to each other and to the original nucleus as regards 

 their chromosome complements; in other words, somatic mitosis is equa- 

 tional. Furthermore, since all the nuclei of the body are normally derived 

 through such mitoses from a single nucleus, each of these nuclei contains 

 descendants of all the chromosomes present in the first nucleus of the 

 series: the somatic nuclei are all alike in their chromosome complements, 

 barring, of course, the effects of occasional aberrant chromosome behavior 

 and other alterations to be described later. The great theoretical impor- 

 tance of these features of somatic nuclear behavior will be apparent when 

 we take up the application of cytological phenomena to the problems of 

 heredity and development. 



Cytokinesis, or the division of the cytosome, follows mitosis quickly 

 in most tissues and tends to separate the various elements — cytoplasm, 

 plastids, chondriosomes, and vacuoles — more or less equally according to 

 their arrangement in the protoplast. Occasionally some of these elements 

 are passively divided. The daughter cells are initially similar in architec- 

 ture and functional capacity. The course of their further differentiation 

 depends largely upon their position in the developing whole. 



