Papers from the Marine Biological Laboratory at Tortugas. 213 



Finally there are two cytoplasmic structures which aid in distinguishing 

 the young apyrene spermatoblasts. The first of these is a very evident, 

 darkly staining granule, whose derivation is unknown. Its position varies; 

 it may lie just beside the nucleus or nearer the periphery, but it is always 

 found in the proximal half of the cell. It usually appears to be surrounded 

 by a clear area (figs. 8 to 12). Occasionally a cell is found which has two 

 Of three of these granules in it. The other structure is the centrosome. 

 This appears at first as a distinct centriole with a few faint rays and it lies 

 between the nucleus and the base of the cell (fig. 8). In other spermato- 

 blasts, which are still so young that they might be confused with the sper- 

 matogonia, it may be seen that the centriole has divided into two and that 

 these are surrounded by a sphere (fig. 9). That centrosomes do exist in 

 the spermatogonia is proved by their appearance in mitoses, but the writer 

 has never been able to identify them positively in the resting cells. The 

 distinct appearance of the centrosomes (centrioles) in the young spermato- 

 blasts is not at all surprising when it is borne in mind that their later history 

 is quite complicated and that they play a very important part in the develop- 

 ment of the apyrene spermatozoa. The other bodies are never found in the 

 cytoplasm of the spermatogonia. The eupyrene spermatogonia greatly 

 outnumber the apyrene spermatoblasts and, with the basal nuclei, they 

 occupy the space next to the wall of the testis except here and there where 

 they have been displaced by groups of the spermatoblasts. These groups 

 occur at irregular intervals and the cells composing them vary not only in 

 number but also in age. 



The development of the apyrene spermatozoa from these spermatoblasts 

 takes place, as has already been indicated, without a single division of the 

 cell. The first period of this development covers a gradual growth of the 

 spermatoblast, resulting in a tremendous increase in the size of the cell. 

 This statement applies not only to the nucleus and cytoplasm, but also to 

 the centrosome; the latter comes to be the most striking and interesting 

 element in the cell. 



The structure of the primitive spermatoblasts has been described. 

 They are irregular in outline and are not pear-shaped or stalked as are the 

 oligopyrene spermatocytes of Paludina (fig. 8). Usually, as they increase 

 in size and are crowded away from the wall of the lobule by the formation 

 and growth of younger spermatoblasts, they are set free and tend to become 

 round (fig. 12). Occasionally a cell which has almost attained its full 

 growth is seen to have retained a connection with the syncytium by means 

 of a stalk, even after it has been crowded away (fig. 13) ; such a cell presents 

 a close resemblance to the oligopyrene spermatocytes of Paludina. In 

 some few instances two or three fully grown spermatoblasts may be seen 

 lying side by side close to the wall of the lobule and only separated from it 

 by a thin layer of syncytium. These cells were the last ones to be formed 

 at that point and have undergone their entire growth without suffering 

 displacement. Both in the cases of these and of the stalked cells there is 



