HISTOLOGY AND CYTOLOGY 367 



the -chromosome groups at the two poles into the condition of the 

 resting nucleus, with its membrane and reticulum restored. This 

 may be regarded as being brought about by a process that is practi- 

 cally the reverse of the changes occurring in the prophase. It is 

 subject to considerable variation, and in the case of rapidly dividing 

 cells, such as we find in developing animals or tissues, may be 

 practically omitted. The centrosome of each nucleus may divide 

 into two during the anaphase, and all that is necessary is for the 

 chromosome group at the end of the anaphase to rotate through an 

 angle of 90 so as to constitute a typical amphiaster practically 

 at once. The stages of the telophase are passed through if the cell 

 returns to a normal non-dividing condition, and the above variations 

 may be regarded simply as short cuts to enable cell division to 

 proceed at a rapid rate. While the nuclear reconstruction is pro- 

 ceeding, or very sfiortly after it is completed, the cytoplasm of the 

 mother cell is cut into two and two daughter cells are produced. 



Certain points in connection with mitosis need to be 

 emphasised. In the first place we see that it is an elaborate 

 mechanism which has as its result the careful collecting up of the 

 chromatin into a number of definite bodies, the chromosomes ; each 

 of these is split into two, and a half goes to each daughter cell. 

 Thus not merely is the chromatin content of the nucleus halved in 

 amount, but it is first sorted out and halves of the individual units 

 distributed to the new cells, suggesting that the halving is also 

 qualitative. It would appear from the universality of the occurrence 

 of mitosis and the consistency of its result, that it is necessary for 

 this material to be carefully dealt with, and so apparently the chro- 

 matin itself must be of great importance to the subsequent activities 

 of the resulting cells. All other cytological evidence tends to confirm 

 this conception of the functional value of chromatin. 



Numerous observations on many species of animals have also 

 revealed the fact that in any one species there is a remarkable 

 constancy in the number and variety of chromosomes produced. 

 Thus for each species there is a fixed number of chromosomes, 

 and they are arranged in a fairly constant manner in all the cell 

 divisions of the body, no matter what organ or tissue they are going 

 to form. This constant number is known as the somatic or diploid 

 number. 



It will be noted that this constant number refers to the 

 cells of the body, and the description of mitosis given is similarly 

 applicable to the somatic cells. A modified form of such cell 

 division is encountered in the history of the germ cells, both male 

 and female, in practically all cases. As a result of this modification 

 we find that the mature germ cells contain only half the number of 



