46 bulletin: museum of comparative zoology. 



plasm. Such is also the case in the smaller spermatocytes, but the 

 change which occurs is entirely different. In these, as the centrosome 

 moves toward the nuclear membrane, the archoplasm, which has hereto- 

 fore been arranged in a zone surrounding the nucleus, now becomes 

 entirely collected upon one side of this vesicle (the side occupied by the 

 centrosome, Fig. 57). This mass of reticular archoplasm does not be- 

 come dissolved at once in the hyaloplasm (Fig. 58), as in the large 

 spermatocytes, but, at the time of the migration of the centrosomes, 

 gradually disintegrates (Fig. 59) and becomes converted into the astral 

 systems surrounding these bodies. Fragments of the archoplasm are 

 often present in the cell up to the time of the dissolution of the nuclear 

 membrane. 



In the very early prophase, before the centrosome has left the mass 

 of archoplasm and moved to the nuclear membrane, the nucleus already 

 shows signs of activity (Figs. 57, 58). The chromatin segments arise 

 from the karyosphere and go through stages similar in a general way to 

 those in the larger type. However, there are some slight differences. 

 The process of tetrad formation requires much less time than in the 

 large type, and by the time the centrosomes have begun their separa- 

 tion along the nuclear membrane, this process is nearly completed 

 (Figs. 59, 61). The segments as they arise from the karyosphere are 

 much shorter and thicker than in the larger spermatocytes (Fig. 5S), 

 and this fact causes the resulting tetrads to resemble much more the 

 typical structures seen in the spermatocytes of insects. As in the largo 

 cells, the tetrads are of diverse form, but are all referable to one class, 

 of which the cross-shaped figure is the type (Fig. 60, a). The more 

 common variations of this type are the double-V figure as shown in 

 Figure 60, a. It can be readily seen how these forms are derived from 

 crosses by a comparatively slight alteration. In the ring figure the long 

 arms of the cross are bent around and come into contact, thus forming 

 an apparently closed circle, while at the point of double fission the 

 ends of the four chromatids are easily distinguishable (Figs. 58, 60, b). 

 During the later stages of chromatin transformation the tetrads shorten 

 and become condensed into homogeneous bodies which are normally of 

 a four-lobed form (Figs. 60, c, 62-64). However, as in the other type of 

 spermatocyte, there are variations from this typical form, due to slight 

 natural modifications and distortions (Fig. 60, c). When fully formed it 

 is seen that the chromosomes of the small spermatocytes are slightly 

 smaller than those of the large type, but when the size of the cells is 

 considered they are relatively larger. 



