THE REDUCTION OF THE CHROMOSOMES 



233 



known as the zygotene, zygonema, synaptene, or amphitene stage. Usually 

 before the union is complete the nucleus enlarges somewhat and the 

 threads contract, forming a tight knot at one side of the nucleus. This 

 stage, formerly called synapsis (see p. 255), is now more properly known as 

 synizesis. The pairing threads come into very close association during 

 synizesis, which, though variable in time, usually ensues at about this 

 stage. When the closely paired threads recover from the synizesis con- 

 traction they extend more uniformly throughout the nucleus ("open 

 spireme"), and are now seen to be much thicker (pachytene; pachynema). 

 In many cases they may again contract into a 

 loose knot with loops extending from it ("second 

 contraction"). As they continue to decrease in 

 length and increase in diameter the members of 

 each pair twist more or less tightly about each 

 other for a short time (strepsitene; strepsinema; 

 diplotene). Eventually they become very short 

 and thick, and the various pairs (gemini; 

 bivalent chromosomes), present in the haploid 

 or reduced number, lie scattered throughout 

 the nucleus (diakinesis}. The two components 

 of each geminus may now separate slightly at 

 one or both ends or at the middle, which gives 

 them the form of Ys, Vs, Xs, and Os. The 

 bivalent chromosomes are now fully formed and 

 ready to take their places on the spindle, which 

 soon forms. 



In the case of the animal egg the "growth 

 period" introduces a complication. In the 

 sporocytes of plants and the spermatocytes of 

 animals there is some enlargement of the cell 

 and nucleus during the stages just described, 

 but the chromosomes pass directly from the 

 strepsinema stage to diakinesis. During the 

 relatively enormous growth of the oocyte on the other hand, 

 the chromosomes, which have usually reached the strepsinema stage 

 when the enlargement begins, become greatly modified in form. 

 Their achromatic framework takes the form of fine threads extending 

 out in all directions, giving the chromosome an irregular brush- 

 like form (Fig. 86, C, D), while the chromatic substance either may 

 flow into the nucleolus, leaving the chromosome framework uncolored and 

 very difficult to observe, or by loss of its staining capacity through chem- 

 ical change it may disappear from view completely. As the growth 

 period comes to an end, however, the original staining capacity returns 

 and the chromosomes again assume the compact form and pass into the 

 diakinesis stage. 



FIG. 85. Parasynapsis in 

 Phrynotettix magnus. 

 A, leptonema; x, sex- 

 chromosome. B, conjuga- 

 tion of "chromosome A." 

 Portions of other uncon- 

 jugated threads and one 

 other bivalent also present 

 in section. X 2000. 

 (After W enrich, 1916.) 



