Papers from the Marine Biological Laboratory at Tortugas. 231 



is full of contradictions. Thus, for instance, Cerithium should perhaps 

 precede Conus, since in the former one chromosome is retained in the apyrene 

 spermatozoon for a longer time than the others (Stephan, '03a), while in 

 Conus all the chromatin of the apyrene spermatocyte degenerates simul- 

 taneously. Again, when considered from the standpoint of the differ- 

 entiations of the centrosome and the development of a motor apparatus, 

 Conus should be placed at the end of the list, for here the apyrene spermat- 

 ozoa are totally immotile. Vermetus has been placed before Strombus 

 because of the formation of chromosomes and of an abortive spindle in 

 the development of the apyrene spermatozoa of the former; but the order 

 could well be reversed when it is borne in mind that, in the development of 

 the bundle of axial fibers of the atypical spermatozoon, Strombus resembles 

 Pdhidina much more than does Vermetus. The truth of the matter is that 

 we do not have sufficient data upon which to formulate such a series. In 

 the writer's opinion, the evidence is not clear enough for us to draw correct 

 inferences as to whether or not we are dealing with a series of retrogressions 

 from the atypical spermatozoa. 



While it has been shown that the atypical spermatozoa, in all those forms 

 in which they have been studied, show certain fundamental similarities, 

 each one also has certain differences peculiar to itself. One of the greatest 

 of these differences, perhaps, is that in regard to the origin of the apyrene 

 spermatozoa of Strombus. Nevertheless, in spite of such differences, it is 

 the writer's opinion that they are all essentially the same and are derived 

 from similar elements of the testis — that is, they are modifications of the 

 accessory cells of the testis, just as are the nurse-cells in the testis of Lit- 

 torina. It is suggested that the explanation of the differences between 

 them is to be found in the manner in which the differentiation of the primi- 

 tive cells of the sexual gland takes place into the various elemicnts of the 

 testis. So, in the Pulmonates there is a sharp differentiation of the primi- 

 tive sex-cells into the true sex-cells and into the accessory cells, the basal 

 nuclei with their surrounding syncytium. Here the latter never become 

 differentiated into independent cells, although they undergo changes in 

 their size and shape (Platner, '85). Among the Prosobranchs, in Littorina, 

 the writer has never observed the same tremendous swelling of the basal 

 nuclei as may be seen in the ovotestis of Planorbis, but, instead, certain of 

 them give rise directly to the nurse-cells to which the spermatozoa are 

 attached. In Strombus those elements which give rise to the atypical 

 spermatozoa are differentiated from the true sex-cells along with the basal 

 nuclei. The latter never give rise to nurse-cells of any kind. In forms such 

 as Paludina and Murex, the elements which give rise to the atypical sper- 

 matozoa are not differentiated from the true sexual elements along with 

 the basal nuclei, but only after the spermatogonia have been formed. In 

 Paludina the formation of nurse-cells has never been observed by the writer 

 nor have they been described by Meves, but the basal nuclei undergo 

 changes which are very similar to those which occur in the ovotestis of 



