MATURATION OF GAMETES 135 



The foregoing statement regarding disjunctional and equational divisions 

 should be considered in the Hght of the phenomenon of crossing over. In 

 the latter process, a gene or groups of genes may pass from one chromatid 

 to the other and vice versa during their association at the four strand stage 

 (fig. 70). In the region of the centromere or kinetochore (i.e., the point) 

 of the achromatic, spindle-fiber attachment) and nearby regions, cross overs 

 are thought not to occur (fig. 70, kinetochore). Consequently, in the regions 

 of the kinetochore, the statements above regarding disjunctional and equa- 

 tional divisions of the chromosomes appear to be correct. However, the terms 

 disjunctional and equational may mean little in other regions of the chromo- 

 somes of a tetrad during the meiotic divisions. For example, let us assume as 

 in fig. 70 (see also fig. 67), that we have chromatids A and A', B and B', 

 A and B representing the original homologues or synaptic chromosomes which 

 have divided into these chromatids respectively. Then during the tetrad stage 

 of association or slightly before, let us assume that there has been a crossing 

 over of genes from chromatid A to chromatid B and from chromatid B to 

 chromatid A in a particular area (fig. 70A). (It is to be observed that chroma- 

 tids A' and B' are not involved in this particular instance.) Further, let us 

 assume that AA' and BB' as a whole are separated at the first maturation 

 division, the kinetochore and immediate regions would represent a disjunc- 

 tional division, but for the particular area where crossing over is accomplished, 

 the division would be equational (fig. 70A, B; central portions of chromatids 

 A and B in fig. 70B). Thus, it would be for other regions where cross overs 

 may have occurred. Other cross-over possibilities are shown in fig. 70C-F. 



c. Stages of Chromatin Behavior During the Meiotic Prophase in 



Greater Detail 



The following five stages of chromatin behavior within the prophase nucleus 

 during meiosis are now in common usage. They are based on the stages 

 originally described by H. von Winiwarter, '00. The substantive form is pre- 

 sented in parentheses. 



1) Leptotene (Leptonema) Stage. The leptotene stage (figs. 69, 71) repre- 

 sents the initial stage of the meiotic process and is seen especially well in 

 the spermatocyte. At this time the nucleus of the differentiating germ cell 

 begins to enlarge, and the diploid number of very long, slender chromatin 

 threads make their appearance. (Compare "resting" and leptotene nuclei in 

 figs. 69, 71.) The chromatin threads may lie at random in the nucleus or 

 they may be directed toward one side, forming the so-called "bouquet" con- 

 dition (fig. 69, leptotene stage) . The nucleolus is evident at this time (fig. 7 IB) . 



2) Zygotene or Synaptene (Zygonema) Stage. The zygotene stage (figs. 69, 

 71, 85) is characterized by a synapsis of the chromatin threads. This synapsis 

 or conjugation occurs between the homologous chromosomes, that is, the 

 chromosomes which have a similar genie constitution. Synapsis appears to 



