258 



INTRODUCTION TO CYTOLOGY 



(Fig. 152). These threads soon undergo a marked shortening and com- 

 monly become noticeably thicker; hence this is called the pachynema 

 stage. The pachynema threads may lie irregularly in the nucleus, or they 

 may retain the bouquet type of orientation for a time. In suitable 

 preparations their bivalent nature is obvious in their distinct doubleness. 

 Moreover, the chromomeres and other structural features show a close 

 correspondence in the two synapsed threads: the pairing tends to be 

 carried out very precisely, part for part. Closer examination, at least 



r 



a 





«^— 



m 



R 



Q JfJ 



/ 



£" 



h 



Fig. 151. — Stages in first meiotic mitosis in Tidipa. a, zygonema (leptonema threads 

 synapsing laterally). 6, single pair of zygonema threads, c, pachynema, d, diplonema. 

 e, early diakinesis. /, late diakinesis. g, diakinesis, showing all of the 12 tetrads, h, 

 early anaphase, showing tetrads beginning to disjoin into dyads, i, later anaphase. 

 {After Newton, 1927.) i,Sv 



late in the pachynema stage, reveals the fact that each of the synapsed 

 threads is double as the result of splitting. Hence each late pachynema 

 thread consists of four chromatids; it is a tetrad. The origin of this split 

 is very difficult to determine. It is said to appear first during late pachy- 

 nema or in the immediately following stage by several observers,^ but 

 others, as already pointed out, find evidence for it at earlier stages. This 

 latter evidence falls in well with that for the presence of two chromone- 

 mata in each chromosome in somatic telophases, but it should not be 

 assumed without adequate evidence that the somatic and the last pre- 

 meiotic mitoses are in all respects alike. 



^ E.g., Newton (1927) and Newton and Darlington (1929) on Tulipa; Darlington 

 (1929c et seq.) on Hyacinihus, etc.; Belling (1931a6) on Allium and Lilmm; Gelei (1921) 

 on Dendrocoelum, Janssens (1924) on Stethophyma (Mecostethus); and Huskins (1932) 

 on Trillium. 



