232 FOUNDATIONS OF BIOLOGY 



are in ordinary mitosis. But, and this is the crucial point, 

 in the early anaphase the members of each pair are separated, 

 one sy nap tic mate going to each pole of the spindle. Thus 

 each of the daughter cells SECONDARY SPERMATOCYTES 

 receives half the total number of chromosomes that were 

 present in the primary spermatocyte or the somatic cells. 

 The essential difference between this type of mitosis (REDUC- 

 TION DIVISION) and that involved in other nuclear divisions 

 (EQUATION DIVISIONS) lies in the separation of entire chromo- 

 somes (synaptic mates) instead of the splitting of each chro- 

 mosome. Both the secondary spermatocytes now divide by 

 typical mitosis, thus distributing to each of the resulting cells 

 (SPERMATIDS) half the somatic number of chromosomes. The 

 spermatids are presently transformed into sperm and thus 

 each spermatogonium with eight chromosomes gives rise to 

 four sperm with four chromosomes apiece. (Fig. 125.) 



4. agenesis 



The maturation of the egg, as already intimated, follows 

 the same plan as that of the sperm, and the reduction of the 

 chromosomes is the same. Such modifications as occur are 

 related to the fact that the egg is usually a relatively large 

 passive cell stored with nutritive materials for use during the 

 developmental process, while the sperm is among the smallest 

 of cells essentially a nucleus surrounded with a delicate 

 envelope of cytoplasm. Accordingly it is only necessary to 

 emphasize that the growth period of egg formation, in which 

 the OOGONIUM becomes transformed into the PRIMARY 

 OOCYTE, is characterized by a much greater increase in size 

 than is the case in the corresponding period in spermato- 

 genesis; and that both of the ensuing cell divisions (one a 

 reduction and the other an equation division) involve very 

 unequal divisions of the cytoplasm. Thus one SECONDARY 



