MATURATION OF GAMETES 137 



begin most often at the ends of the threads and progresses toward the middle 

 (fig. 67, zygotene). At this stage the chromatin threads may show a strong 

 tendency to collapse and shrink into a mass toward one end of the nucleus 

 (fig. 85C, D). This collapsed condition, when present, is called synizesis. 

 The zygotene stage gradually passes into the pachytene condition. 



3) Pachytene (Pachynema) Stage. Gradually, the synapsis of the homolo- 

 gous chromosomes becomes more complete, and the threads appear shorter 

 and thicker. The contracted threads in this condition are referred to as 

 pachynema (figs. 69, 71, 85E). The nucleus in this manner comes to contain 

 a number of bivalent chromosomes, each of which is made up of two homolo- 

 gous mates arranged side by side in synaptic union, known technically as 

 parasynapsis. (Telosynapsis probably is not a normal condition.) Conse- 

 quently, the number of chromosomes now appears to be haploid. Each pachy- 

 tene chromosome (i.e., each of the pair of homologous chromosomes) grad- 

 ually divides equationally into two daughter thread-like structures, generally 

 referred to as chromatids. The exact time at which division occurs during 

 meiosis is questionable. The entire group of four chromatids which arise from 

 the splitting of the synapsed homologues is called a tetrad. 



4) Diplotene (Diplonema) Stage. In the diplotene stage (figs. 67, 69, 71, 

 85F, G), two of the chromatids tend to separate from the other two. (See 

 fig. 70A, C, E.) The four chromatids in each tetrad may now be observed 

 more readily, at least in some species, because the various chromatids of 

 each tetrad show a repulsion for one another, and the chromatids move 

 apart in certain areas along their length. This condition is shown in both the 

 male and female meiocyte, but in the latter, the repulsion or moving apart 

 is carried to a considerable degree and is associated with a great lengthening 

 and attenuation of the chromatids. (See fig. 67.) In the female meiocyte at this 

 stage, the chromosomes become very diffuse and are scattered throughout 

 the nucleus, somewhat resembling the non-mitotic condition (figs. 71F-I; 

 72B-E). The peculiar behavior of the chromosomes and nucleus of the oocyte 

 in the diplotene stage of meiosis is described more in detail on p. 141. 



Although there is a tendency for the chromatids to widen out or separate 

 from each other at this time; they do remain associated in one or more regions. 

 In these regions of contact, the paired chromatids appear to exchange partners. 

 This point of contact is called a chiasma (plural, chiasmata). Hence, a 

 chiasma is the general region where the chromatids appear to have exchanged 

 partners when the tetrad threads move apart in the diplotene state. (See fig. 

 70, chiasmata.) 



5) Diakinesis. The diplotene stage gradually transforms into the diakinesis 

 state (figs. 67, 69, 72F, 85H) by a process of marked chromosomal con- 

 traction. There also may be an opening up of the tetrads due to a separation 

 of the homologous mates in the more central portions of the tetrad, with the 

 result that only the terminal parts of the chromatids remain in contact. This 



