MEIOSIS 



257 



of shrinkage is claimed to be natural. Eventually the synapsis of homol- 

 ogous threads becomes complete, and the tendency to collapse, which 

 renders many cytological preparations worthless, is no longer so evident. 



6^ 



(W(3 



9 10 11 12 



Fig. 150. — Semidiagrammatic representation of the principal stages of meiosis. 1, 

 leptonema; threads present in the diploid number (6). 2, zygonema; synapsis of homo- 

 logous chromosomes in progress. 3, pachynema. 4, diplonema; note chiasmata. 5, 

 diakinesis; chiasmata fewer; matrix developed. 6, metaphase /. 7, anaphase /; tetrads 

 separated into dyads. 8, telophase /. 9, prophase II; chromatids remain associated 

 at spindle-attachment regions. 10, metaphase //. 11, anaphase //. 12, telophase //; 

 each nucleus has the monoploid number (3) of chromatids. The split which is to become 

 effective in the first postmeiotic mitosis is indicated in portions of the chromonemata in 

 8-12. Cf. Fig. 54. 



The nucleus now contains the reduced number of bivalent threads, 

 each of which represents two homologous chromosomes in synaptic union 



