STUDIES ON SEX IN CREPIDULA 19 



multiplication of the latter; for the primordial egg cells are found 

 only in the germinal layer, while the spermatogonia fill the lu- 

 men as it grows larger and larger. Actually, however, the type 

 A cells increase in number, and their presence is more noticeable 

 than at any other time in the history of the gonad up to the 

 initiation of the female phase. Their prominence is partly due 

 to the fact that their chromatic bodies become more distinct 

 and stain with greater intensity than at other times. There is 

 not the slightest indication that these cells take part in the 

 formation of any of the products of the testis, however, and it 

 seems probable that they undergo a limited amount of division 

 which is correlated with the enormous development of the gonad 

 as a whole. Rarely one may find their large and characteristic 

 division figures at this time (fig. 9). 



The cells filling up the lumen of the gonad (fig. 3), all alike 

 so far as visible characteristics go, follow two divergent paths 

 of development after the close of the multiplication period; one 

 of which leads to the development of the typical sperm capable 

 of fertilizing the egg, while the other ends in the formation of the 

 atypical sperm, which in Crepidula are of the 'apyrene' variety. 

 Following the terminology of Meves ('03) the word 'apyrene' is 

 used to describe atypical spermatozoa which in the adult con- 

 dition contain no vestige of chromatic matter; 'oligopyrene' 

 refers to those which contain a little chromatin; the true fer- 

 tilizing elements are known as the 'eupyrene' spermatozoa. 



Spermatogenesis. Historical. A. The typical series. In re- 

 cent times the spermatogenesis of the true sperm of some Proso- 

 branchs has been fully worked out (Meves, '03, on Paludina; 

 Kuschakewitsch, '13, on Conus and Vermetus). The figures of 

 the spermatocytes of the first and second orders, maturation, di- 

 visions and spermiogenesis given by other authors have been 

 compared with the conditions found in the testis of Crepidula, 

 and so far as the observations have gone there are no striking 

 peculiarities which would warrant a detailed description of the 

 eupyrene spermatogenesis of Crepidula plana. The develop- 

 ment of the spermatocytes will be carried only far enough to 

 show their different behavior from the cells which are destined 



