CHAPTER V 

 MITOSIS 



THE growth of all tissues depends primarily upon the increase in the 

 number of cells constituting the tissue. New cells arise only from par- 

 ent cells by division of the parent cells. Since the life of any cell depends 

 upon the presence of a nucleus, this process of division involves a divid- 

 ing of the nucleus. In most cases of cell division the dividing nucleus 

 elongates at right angles to the plane of cytoplasmic division. There 

 are two types of nuclear division. The one is comparatively simple in 

 the number of phases which it presents; the other involves a series of 

 complex nuclear changes. The first is known as amitosis and is con- 

 sidered in another chapter ; the second has been termed by various writers 

 mitosis, karyokinesis, and indirect division. 



The nucleus of a given species always undergoes in its mitosis a 

 definite series of structural changes. The mitoses for various species 

 present considerable variation. As mitosis has to do primarily and 

 essentially with an equal division of the chromatin of a mother nucleus 

 between two daughter nuclei, there is encountered less variation in the 

 structural phases assumed by the chromatin than in the other structures 

 concerned with mitosis. 



The series of chromatin phases and their order of sequence is dia- 

 grammatically represented by Figure 20, A to 7. The chromatin, which, 

 in a resting nucleus, is more or less generally distributed within the 

 nucleus as granules of chromatin (Fig. 20, A), is assembled to form in 

 most cases a chromatin thread known as the spireme (Fig. 20, B). The 

 nucleus now elongates, and in the higher forms the nuclear membrane 

 at this stage disappears. The spireme thread breaks into a number of 

 fragments. These may be rounded to rod-shaped bodies. Each chro- 

 matin segment is called a chromosome. In our diagram we have shown 

 at C six chromosomes derived from the spireme. // is highly probable 

 that the number of chromosomes in a nucleus is constant for a given species. 

 For example, with the breaking of the spireme of a somatic nucleus from 

 man, sixteen chromosomes will arise; the nucleus of Ascaris megalo- 

 cephala var. bivalens represents four chromosomes. In certain mitoses 

 the number of chromatin segments may vary from the specific number. 

 It seems to be supported by evidence that when such variation occurs, 



