I3O LOUISE B. WALLACE. 



are closely pressed together the double nature of this nucleolus- 

 like mass is easily demonstrated after long extraction in iron- 

 alum (Figs. 46 and 47). As to what happens during this proc- 

 ess of contraction I am wholly in the dark but later the nuclear 

 cavity is fully occupied by a delicate reticulum upon which the 

 chromatin granules are distributed in such a finely divided con- 

 dition that they show very slight affinity for staining reagents 

 (Fig. 48). The centrosome has divided into a proximal and dis- 

 tal portion and the proximal centrosome has moved some dis- 

 tance over the nucleus or has possibly entered into its interior. 

 During its passage it gives rise to an intra-cellular filament which 

 connects the proximal and distal centrosomes. The extra- 

 cellular axial filament is now larger and its vesicles have in- 

 creased noticeably in size. The accessory chromosomes are no 

 longer inclosed in a vesicle but unite side by side into a single, 

 elongated rod which leaves its former position at the distal pole 

 and travels to the central pole in a line nearly or quite parallel 

 with the cell-axis (Fig. 49). Following this stage the nucleus 

 changes in outline, becoming somewhat pear-shaped and the 

 proximal centrosome, which has become large and irregular in 

 form, has passed over about one half of the length of the 

 nucleus. The chromatin shows a tendency to collect at one 

 side of the nucleus to form the chromatin plate. In half of the 

 spermatids the fused or nearly fused accessory chromosomes 

 occupy the center of the chromatin plate, extending from the 

 central to the distal pole of the nucleus, or, in other words, 

 from the anterior to the posterior end of the rapidly forming 

 spermatozoon head (Figs. 50 and 51). The chromatin plate in- 

 creases in size until all of the chromatin reticulum is involved ;. 

 the whole nucleus becomes much longer than broad with the 



o 



extremities slightly curved. The sister spermatids now separate 

 completely, the rupture occurring on each side of the " connect- 

 ing-body" when it is present. Figure 53 shows a somewhat 

 later stage where the transformation is complete. The pear- 

 shaped nucleus of the spermatid has been transformed into the 

 crescent-shaped head of the spermatozoon and the chromatin is 

 so compact that the whole head has a dark, grayish hue after 

 long extraction in iron-alum. Even at this late stage the 



