76 EDWARD DRANE CRABB. 



asp era. He says : " Finally, I believe that the nucleus of the in- 

 different germinal epithelial cell may be stimulated by a variety 

 of external agencies to tend towards one sex." His general 

 opinion with regard to the condition in Helix in later papers (espe- 

 cially '19, p. 431) shows further that he believes that many germ- 

 nurse-cells de-differentiate to form spermatocytes. As a result 

 of further investigation on this subject ('22) he says: ' The 

 reason for the passage of the indifferent epithelial cell, either to 

 the oogonium or spermatogonium, is at present unknown. Nu- 

 trimental conditions probably do not represent the real causal 

 state." Buresch ('12) is satisfied that no cells ever de-differ- 

 entiate. The work of Merton ('24) on the amoeboid movements 

 of ciliated-, nurse- and germ-cells and the phagocytic propensities 

 of egg cells in Planorbis may indicate a way leading to the solution 

 of this problem. Whether the germ-cells are derived from " in- 

 different epithelial substance " or from a " syncytium " or are 

 " de-differentiated ' : from indifferent epithelial cells, or agents 

 which cause early germ-cells to develop into ova instead of sperma- 

 tozoa or v ice versa is a question not within the scope of this paper. 



The germ-cells continue to develop side by side until they be- 

 come detached from the wall of the lumen. After this they re- 

 main associated until they reach the anterior part of the her- 

 maphrodite duct where it is possible that their relative numbers 

 may be reduced by a sort of segregation of the sperms and eggs 

 shortly before reaching the oviduct and vas efferens. From the 

 fact that it has been shown that in most species of hermaphroditic 

 land and fresh-water snails the germ-cells develop side by side in 

 the acinus, we know that this is not peculiar to Lymncea. 



The migration of the germ-cells is erratic inasmuch as it is 

 begun in different stages of development in individuals of the same 

 kind. For instance, some of the advanced eupyrene spermatids 

 loosen themselves from the Sertoli cells, speedily ripen and thus 

 begin the migration free in company with other spermatids, while 

 in many other instances the Sertoli cells break loose from Ancel's 

 layer and begin the migration with scores of normal advanced 

 spermatids attached. In most instances these spermatids drop off 

 before the nurse-cell reaches the hermaphrodite duct proper. At 

 this time these spermatids appear as in Fig. 8. Frequently masses 



