34 CYTOLOGY 



CHAP. 



are nine of the thick bands, thus justifying the statement made above 

 that there were eighteen of the thin threads in the leptotene nucleus. 



The nine thick chromosomes now present, having been formed each by 

 the conjugation of two homologous chromosomes, are said to be bivalent, in 

 contra-distinction to the separate, unconjugated or univalentchromosomes. 



Fig. 14, H, is a later pachytene nucleus. In this the shortening and 

 thickening of the chromosomes, which proceeds throughout the meiotic 

 as throughout the somatic prophase, has progressed further. 



At the stage shown in Fig. 14, I, a process the reverse of what 

 occurred in the zygotene stage is taking place, the bivalent chromosomes 

 splitting into their two constituents again. Hence this phase is called 

 the diplotene stage, or, since the members of each pair are often con- 

 spicuously twisted round one another, the strepsitcne stage. The polar 

 orientation of the chromosomes is now less pronounced, and by the stage 

 shown in Fig. 14, J, it has quite disappeared. By this time the separation 

 of the two constituents of each pachytene bivalent has proceeded consider- 

 ably further, and indeed is complete in the case of some pairs. Others 

 remain attached at one end, giving rise to U-shaped figures, or at both 

 ends producing figures shaped Q, or if twisted to figures of 8. Others, 

 again, may remain united at their middles and separate at their ends, 

 forming X or ^(-shaped figures. 



The nucleus has now commonly attained its greatest volume, and the 

 chromosomes are characteristically distributed round its periphery, 

 immediately beneath the nuclear membrane. This is the phase called 

 by Hacker diakinesis, and is a conspicuous stage in most gametogeneses. 

 Soon after this the nuclear membrane disappears, and the chromosome 

 pairs lie in the cytoplasm (Fig. 14, K). 



Thus the long series of changes which the nucleus passes through 

 from the first appearance of the leptotene threads to the point at which 

 we have now arrived, all take place in a long drawn-out mitotic prophase, 

 namely, the prophase of the first meiotic division, and are succeeded by 

 the metaphase and anaphase of this division (Fig. 14, L, M). In these, 

 the chromosomes which paired in syndesis and disengaged themselves 

 again in the diplotene stage are finally separated, one member of each 

 pair going into one daughter nucleus, and the other member into the 

 other. This division is therefore the true meiotic or reduction division, 

 since through its agency two secondary spermatocyte nuclei, each con- 

 taining nine chromosomes, are formed from one primary spermatocyte 

 nucleus containing nine pairs, or eighteen, chromosomes. 



It wiU be noticed that each of the separating chromosomes in the 

 anaphase is itself split along its length. This is an exaggeration, 

 commonly found in meiosis, of the tendency (discussed in Chapter I.) of 

 chromosomes to exhibit already in the anaphase that division into two 



