DAVIS: SPERMATOGENESIS. ' 95 



•61). Apparently the centrosomes are already some distance apart 

 when first distinguishable, and continue to migrate around the nucleus 

 until they come to lie opposite each other. The nuclear membrane 

 then gradually breaks down, first disappearing in the vicinity of 

 the asters. As the nuclear membrane disappears the astral rays extend 

 into the nucleus and become attached to the chromosomes. Figure 

 62 shows a somewhat later stage, after the spindle is fully formed, but 

 before the chromosomes are drawn into the equatorial plane. The 

 autosomes have now become homogeneous and have perfectly smooth 

 contours, but they retain, in general, the characteristic forms seen in 

 the earlier stages. Figure 63 shows the mctaphase of the first matura- 

 tion division. As the chromosomes become drawn into the equatorial 

 plane, the rod-shaped forms are arranged with their long axes parallel 

 to the axis of the spindle, the mantle fibers being attached at each end 

 (Plate 7, Fig. 164). Thus the spindle fibers from one pole are attached 

 to one of the univalent components of the bivalent autosomes, those 

 from the other pole to the other uniA'alent component. The cross- 

 shaped autosomes become so arranged that one pair of arms lies along 

 the spindle while the arms of the other pair project away from the axis 

 of the spindle and, at the same time, make a considerable angle with 

 each other, so that in a polar view they appear V-shaped, the apex of 

 the V being directed toward the spindle. In these autosomes it is 

 impossible to determine whether the plane of union of the univalent 

 components is at right angles to the axis of the spindle or parallel to it, 

 but from analogy with the other types it is probably at right angles. 

 In the case of the closed rings and crossed loops the evidence is more 

 satisfactory. These chromosomes become so arranged on the spindle 

 that the free end of each univalent component is attached to mantle 

 fibers from the opposite pole of the spindle, while the apex, which 

 represents the point of union of these components, projects away from ' 

 the spindle (compare Figs. 179, 180). 



In the metaphase of the first maturation division there are always 

 twelve chromosomes, eleven of them being bivalent autosomes, which 

 vary greatly in size. Figure iV is a polar view of the equatorial plate 

 in two adjoining cells and shows that in general the autosomes have 

 the same size relations as the autosome pairs of the spermatogonia, 

 although in the spermatocytes th? relative size of the chromosomes, 

 on account of their irregular form, is not as clearly marked. Inasmuch 

 as the univalent components of each bivalent autosome are of equal 

 size, there seems to be no good reason for doubting that each bivalent 

 autosome is formed by the union of two homologous autosomes of 

 the spermatogonia. 



