16 DUBLIN 



After the separation of the sister spermatids, a resting period 

 of considerable length occurs. The chromatin aggregated into 

 one spherical mass at the telophase, loses in staining capacity 

 and breaks up into small granules which extend throughout the 

 nucleus (Fig. 55). These granules may be of all sizes, and 

 evidently represent the different stages in the successive breaking 

 down of the large mass into its components. The nearly spher- 

 ical cells now begin to elongate along one axis, and the chro- 

 matin definitely localizes itself into two masses at two opposite 

 points along the axis of elongation, leaving only a few scattered 

 granules in the center (Fig. 56). In Fig. 57 the anterior end 

 of the head is clearly to be distinguished from the posterior 

 one. The former is drawn out to a point into which the chro- 

 matin is very densely crowded ; the latter is much rounder and 

 shows distinctly an end knob which, already existing in the 

 preceding stage, is the beginning of the formation of the middle 

 piece. Between the two ends the chromatin granules have 

 come together more closely (Figs. 57-58) and now form a 

 third deeply staining area (Fig. 59). With the further elon- 

 gation, the chromatin masses, before and behind, are pushed 

 out in the direction of the central strip with which they finally 

 come into contact. The chromatin is thus made continuous 

 throughout the head, although open spaces may still be made 

 out (Fig. 60). In the meantime the tail is increased in length, 

 and the whole structure takes on the appearance of the matured 

 spermatozoon. The spaces soon fill up, and the head, now 

 homogeneous, has reached the end of its development. The 

 middle piece however, is still of considerable breadth, nor has 

 the tail anything like its final form. These two structures 

 stand in a reciprocal relation to each other. What the middle 

 piece loses in size, shows in its increased length of the tail. This 

 process continues until the former is reduced to a small deeply 

 staining knob behind the head. In the final changes, not even 

 this persists, for in the great majority of cases the middle piece 

 and head fuse closely together, leaving not even a slight con- 

 striction to indicate the former independence of the two. The 

 mature sperm is thus a composite of two distinct portions, — a 



