MEIOSIS 113 



certain animal oocytes, where the chroniosomc^s at the close of anaphase / 

 immediately become arranged in new spindles for division //. In ordi- 

 nary interkinetic nuclei where the chromosomes can still be seen, the two 

 chromatids of each dyad, although more slender than during the anaphase, 

 continue to remain closely associated at the kinetochore and widened out 

 elsewhere ; hence they tend to appear as X's with arms varying in length 

 according to kinetochore position. 



Cytokinesis does not invariably follow division /. In the micro- 

 sporocytes of many vascular plants and in the meiocytes of certain lower 

 plants the two nuck^i lie in a common mass of cytoplasm and undergo 

 division //, after which the meiocyte is divided simultaneously into four 

 cells (Fig. 101, 4)- In other plant meiocytes, cytokinesis does occur aftei- 



1 2 3 4 



Fig. 83. — Chromosomes in first meiotic division in microsporocytes of Trade scantia. 

 1, bivalent at metaphase about to disjoin; in each half the apparently single thick spiral is 

 actually composed of two chromonemata which represent the chromatids. 2, a, b, c, meta- 

 phase bivalents; the two spiral chromonemata in each of the dyads, which are about to 

 disjoin upward and downward, are separating from each other laterally (a and b are two 

 prints from the same negative). 3, one dyad at anaphase; the two chromatid spirals now 

 lie side by side.- 4, four dyads at anaphase; in each of them the two chromatids remain 

 closely associated only near the kinetochore. (After K. Sax and L. M. Humphrey.) 



I (Fig. 84). In animals the primary spermatocyte is divided into two 

 secondary spermatocytes in which division // then takes place. The 

 primary oocyte divides very unequally to form a minute polocyte (polar 

 body) and the secondary oocyte; division II follows in the latter but not 

 alwaj^s in the former (Fig. 91). Further c,ytological features of these 

 reproductive stages are to be added in the next three chapters. 



Prophase II. — This prophase is much simpler than prophase I. When 

 the transformation of the chromosomes in telophase / has not been carried 

 very far, prophase // consists in little more than their resumption of a 

 more compact form in each of the daughter nuclei of division 7. The two 

 chromatids of each dyad tend to retain in some degree the divergent 

 position they first assumed in anaphase /, so that their association, except 

 at the kinetochore, is usually looser than that observed in somatic 

 prophases. 



Metaphase II and Anaphase II. — The dyads in the two nuclei now take 

 up positions \vith their kinetochores at tin; equators of newly formed 



