232 Papers from the Marine Biological Laboratory at Tortugas. 



Planorhis. In Vermetus and in Conns, too, according to Kuschakewitsch 

 ('13), the atypical spermatozoa are not differentiated until after the forma- 

 tion of the spermatogonia. But in both these forms nurse-cells are also 

 developed and apparently from the basal nuclei; whether or not their 

 development is the same as that of the nurse-cells in Littorina is not known 

 by the writer. In all these forms there is a period in the development of 

 the testis when the true sex-cells are undifferentiated from the accessory 

 elements. It may be that certain differences in the plasma of the primitive 

 cells lead to differences in the manner of differentiation of the accessory 

 elements and these in turn lead to the production of adult structures which 

 differ from each other to a greater or less degree, but all of which show 

 certain fundamental resemblances. 



Besides the characteristics held in common between the atypical sper- 

 matozoa of Stromhus and those of other forms, the former show certain 

 resemblances to the nurse-cells in the testis of Vermetus and of Littorina. 

 While there are no large cells in the testis of Stromhus which could possibly 

 be taken for anything except the apyrene spermatoblasts, there exist a 

 striking resemblance between certain of the developmental stages of these 

 and some of the figures given by Kuschakewitsch ('13) for the nurse-cells 

 in the testis of Vermetus (figs. 104 and 105, Taf. xxiv, and figs. 169 and 170, 

 Taf. XXV, Arch. f. Zellforsch., Bd. x). The writer has shown that nurse- 

 cells are developed in the testis of various species of Littorina which, when 

 they have reached their maximum size, resemble the oligopyrene spermato- 

 cytes of Paludina. Later on, the cells become filled with large secreted 

 bodies, while their nuclei gradually degenerate. In the final stages the 

 latter may still be seen in a very shrunken, degenerated condition lying just 

 beneath the cell-membrane. It is in connection with the formation of these 

 secreted bodies and the corresponding changes in the nucleus that the 

 apyrene spermatozoa of Stromhus resemble the nurse-cells of Littorina. It 

 will be remembered that in the apyrene spermatoblasts, after the nucleus 

 breaks down, chromatic vesicles are formed which gradually increase in 

 size until the formation of the albuminous bodies is begun. From this 

 point on, with the continued formation of the albuminous bodies, they 

 gradually degenerate and are finally all absorbed in the cytoplasm. 



A comparison of the processes which occur on these cells and those 

 which take place in normal secretory cells lead to some interesting consider- 

 ations. Mathews ('99) has shown that secretory activity or, to use his 

 term, hylogenesis, is accompanied in various epithelial cells by certain 

 changes in the nucleus, in that the latter becomes irregular in outline and 

 is displaced towards the base of the cell. In these cells the process is cyclic; 

 after the cell has been exhausted the nucleus returns to its normal condition 

 and the activity begins over again. The changes which the nurse-cells of 

 Littorina undergo at the commencement of their secretory activity are very 

 similar to those which take place in secretory epithelial cells. In the former 

 case, however, the process is not repeated and in every instance it is ac- 



