MATURATION 135 



made out (Figs. 72, 73). This stage is called the contraction 

 phase, or synizesis (McClung) (pachynema, Winiwarter). In 

 some cases synizesis may occur near the beginning of the growth 

 period, throughout , which the nucleus then remains in this" 

 condition. 



When growth of the spermatocyte is completed this knot of 

 chromatin begins to disentangle and the spireme again becomes 

 visible. This later spireme is not continuous, however, but is 

 of the segmented type (Fig. 72), and the number of segments, 

 i.e., of chromosomes, is but one-half the number of chromosomes 

 that went into tJw nucleus at the close of the preceding division. As 

 regards the chromatic structures, this is the essential point in 

 the whole maturation process; the number of chromosomes 

 formed in the prophase 6f the first spermatocyte division is 

 reduced to one-half the somatic number. This numerical 

 reduction of the chromosomes is brought about in most, if not 

 in all cases so far known, by an actual fusion, by twos, of the 

 chromosomes contained in the last spermatogonial nucleus. 

 This fusion of pairs of chromosomes is termed synapsis (Moore, 

 McClung) or synopsis (Hacker) (zygonema, Winiwarter), and 

 the resulting units are thus double or bivalent chromosomes. It 

 seems entirely likely, if not definitely established, that the pairs 

 of chromosomes which come together in synaptic fusion are 

 each composed of one chromosome derived from the male 

 parent, and the corresponding chromosome derived from the 

 female parent, similar in size, and form, and also in function if 

 we assume the fact of chromosomal specificity (Montgomery). 

 These two groups of similar elements came into a single nucleus 

 during the fertilization process which was the beginning of the 

 new individual, whose cells are now preparing for reproduction; 

 they have remained separate throughout the life of this organ- 

 ism until this event, through all the divisions of the ancestral 

 germ-forming cells (Fig. 80). In a certain sense, therefore,_this_- 

 process of synapsis represents the real climax of the whole 



splitting is less complete. One ring is cut through showing two free ends. F. 

 Advanced synizesis. G. Chromosomes appearing after synizesis, shortened and 

 thickened. The ex-conjugant chromosomes (univalent) have separated and show 

 transverse constrictions, preparatory to the second maturation division. 



