22 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The cytosorae of the large type of spermatocytes is of relatively enor- 

 mous size (60 to 90 mi era in diameter), and exhibits in a beautiful man- 

 ner the characteristic structure of the cytoplasm. The nucleus, which at 

 this stage is surrounded by a delicate hut well-defined membrane, is also 

 filled with a regular network of linin fibres, in the meshes and at the 

 nodal points of which are here and there imbedded coarse granules of 

 metaplasm. The entire chromatin of the cell has the form of a spireme 

 or spiremes closely packed about the accessory chromosome (Fig. 19, 

 a-e). This mass of chromatin is the karyosphere. 



The differences from these conditions presented by the small type are, 

 exclusive of size, few and apparently unimportant, but that they are of 

 some significance is demonstrated by comparing the later behavior of 

 the two kinds of cells. They may be briefly summarized as follows : 

 During the growth period, at the time of the rearrangement of the 

 chromatin segments into the karyosphere, the two cells derived from a 

 spermatogonium become separated in such a way that the intercellular 

 bridge formei'ly connecting them is destroyed, the protoplasm of the two 

 cells being no longer continuous. This is perhaps correlated with the 

 fact that, although the two cells remain very close together, and, indeed, 

 are often in contact, development does not continue at exactly the same 

 rate in both. The result is that the two daughter cells from one sper- 

 matogonium, instead of being of the same size and in exactly the same 

 stage of development, present differences which, though usually slight, 

 are often more marked. Frequently cells, though in the same stage of 

 mitosis, show such differences in size as are seen in Figure 57 (Plate 5). 

 On the other hand, two sister cells may be in quite different stages of 

 development (Fig. 156). However, the most striking variations in the 

 vesicle stage are due to the much smaller size of one type and the dis- 

 proportions in the size of the different parts of the cell. As I have 

 elsewhere stated, the spermatocyte of the smaller type varies consider- 

 ably in volume, but the diameter of the average cell is about 35 micra, 

 whereas that of the larger type is about 70 micra. In the smaller sper- 

 matocytes the nucleus is considerably larger in proportion to the amount 

 of cytoplasm than in the larger type, although otherwise similar to it in 

 all essentials. The smaller size of the cytosome seems to result in a 

 smaller amount of archoplasm. The amount of archoplasm may also be 

 influenced possibly by the absence of a connection between the two cells 

 of a pair. 



Another difference exhibited hy the two types of cells concerns their 

 position in the testis. As I have pointed out, the cells of the larger 



