244 ANIMAL BIOLOGY 



without impairing the chromatin heritage from cell to cell. We 

 shall describe first the origin of the sperm, or spermatogenesis, 

 and then proceed to the fundamentally similar origin of the egg, 

 or oogenesis. (Fig. 165.) 



3. Spermatogenesis 



A given spermatogonium, with, let us say, eight chromosomes 

 characteristic of the species, proceeds to increase in size prepara- 

 tory to the first maturation mitosis, and is designated a primary 

 spermatocyte. At the close of the growth period, when this cell 

 is preparing to divide, the chromosomes are arranged in pairs by 

 a process termed synapsis. The number of such pairs will obviously 

 be half that of the chromosome number. The synaptic pairs are 

 then distributed in the equator of the spindle exactly as the single 

 chromosomes are in ordinary mitosis. But in the early anaphase 

 the members of each pair are separated, one synaptic 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. It 

 will be noted that the essential difference between this type of 

 mitosis (reduction division, or meiosis) and that of typical 

 nuclear divisions lies in the separation of entire chromosomes 

 (synaptic mates) instead of the splitting of each chromosome. 

 Reduction thus involves the segregation of synaptic mates in 

 separate cells. 



Both the secondary spermatocytes now divide by typical mito- 

 sis, including chromosomal division, and so each of the resulting 

 cells (spermatids) receives half the somatic number of chromo- 

 somes. The spermatids are presently transformed into sperm, and 

 thus each spermatogonium with eight chromosomes (diploid group) 

 gives rise to four sperm with four chromosomes (haploid group) 

 apiece. (Fig. 165.) 



It may be mentioned in passing that the chromosomal division 

 just described as taking place in the secondary spermatocytes 

 usually occurs precociously in the primary spermatocyte while 

 the chromosomes are in synapsis. Thus each synaptic pair is 

 resolved into a group known as a tetrad, the four components of 

 which are thereafter distributed by the two maturation divisions, 

 and accordingly either or both of these divisions may be involved 

 in segregation. However, for simplicity of exposition we may dis- 



