DAVIS: SPERMATOGENESIS. 99 



ponent of each bivalent autosome must therefore separate from its 

 mate during the first maturation division. In the case of the cross- 

 shaped type it is impossible to determine whether or not the univalent 

 components separate from each other during this division, but it seems 

 as though the division in these forms was the same as in the others. 



The first maturation diAision is, then, a true reduction division in 

 Weismann's sense of the term, since it separates the univalent compo- 

 nents of the bivalent autosomes. 



Figure G5 (Plate 4) shows the beginning of the anaphase (compare 

 also Figure Ma-Mc, p. 96) and Figure 66, the late anaphase of the first 

 division. At the end of the anaphase the chromosomes are collected at 

 the ends of the spindle into two daughter groups, in which the outlines 

 of the elements can be recognized only with difficulty. The centro- 

 somes, which, as in the spermatogoAial divisions, reach their greatest 

 size during the metaphase, have entirely disappeared at this stage. 

 The interzonal filaments form a deeply staining fibrillar mass around 

 the periphery of the region formerly occupied by the spindle. This 

 mass stains much more deeply in a region midway between the chro- 

 matin masses than elsewhere. The explanation, I believe, is that in 

 this region part at least of the mitochondrion, which was collected 

 around the middle of the spindle during the preceding stages, has 

 become mingled with the interzonal filaments. Later, as the constric- 

 tion between the two daughter cells deepens, the interzonal filaments 

 are forced together and finally disintegrate. 



Figure 75 (Plate 5) shows a secondary spermatocyte in the "semi- 

 resting" stage. A nuclear membrane is formed and the autosomes 

 have become partially broken down, but show a more or less distinct 

 V-shape, the space between the two arms being the longitudinal split. 

 Judging from the scarcity of cells in this stage it is of short duration. 

 Figure 76 shows the prophase and Figure 77 the metaphase of the 

 :second maturation division. The more or less curved rod-shaped 

 chromosomes are arranged in pairs around the periphery of the spindle 

 (Figs. Q, R). Each pair represents a V-shaped daughter chromosome 

 of the first division.. The chromosomes show practically the same 

 size relations as the chromosome pairs of the spermatogonia, but on 

 account of their more irregular shape the difference in size is not as 

 evident. During the anaphase of the second division (Figs. 78, 79) 

 one member of each \)r\y passes to each pole of the spindle. Figure 

 80 shows a telophase of the second division. 



Since, during the second maturation division, the autosomes divide 

 along the plane of the longitudinal split which first appeared in the 

 polar spireme, this division is equational. 



