154 GENERAL BIOLOGY 



tetrad behave as chromosomes in the subsequent 

 stages of mitosis. Half of them are pulled to one 

 pole of the spindle and half to the other, and the 

 cell-wall that forms divides the maturing gamete 

 (termed spermatocyte, in the male, or oocyte, in the 

 female) in two. The second peculiarity of the matu- 

 ration mitoses arises from the fact that when these 

 two daughter-cells (spermatocytes or oocytes of the 

 second order) divide again, they do so without the 

 intervening " resting stage " which is elsewhere 

 universal in indirect cell division. The chromatin 

 units have already been segregated into two groups 

 in the first division. They are immediately divided 

 again into two groups, which separate in the two 

 daughter-cells. Four daughter-cells thus result 

 from the division of the first spermatocyte (or 

 oocyte), and, although the latter had, previously, 

 twice the normal number of chromatin units, the 

 number in each of the resulting four cells has been 

 reduced to half the normal number on account of 

 these two rapidly succeeding divisions. This reduc- 

 tion of the chromosomes is of much theoretical inter- 

 est in connection with the subject of heredity. The 

 process seems to be identical in both egg and sperm 

 so far as the nucleus is concerned, but the dif- 

 ference in the behavior of the cytoplasm, in sper- 

 matocyte and oocyte, is very marked. The two 

 cleavages, in the case of the spermatocyte, result 



longitudinally and transversely, to produce the four units of the tetrad. 

 Accordingly, there are half as many tetrads but twice as many chromatin 

 units as the normal number of chromosomes. 



