222 GASTROPODA. 



abstracted from the uterine portion of the common duct in a way 

 similar to that in which the prostatic part of the vas deferens 

 arose from it earlier. These abstrictions take place by means 

 of longitudinal folds which grow into the common canal that arose 

 when the strand became hollow. The albuminiparous gland arises 

 in the form of a number of tubular outgrowths near the proximal 

 end of the uterus (Fig. 103, alb). 



The origin of the male and female ducts through the division of a common 

 rudiment may be demonstrated with some certainty in the various herma- 

 phrodite forms that have been investigated. When we take into consideration, 

 in this connection, that, in the Opisthobranchia, the transmission of the two 

 kinds of genital products takes place through a common duct (Fig. 104 B) 

 and that, in the Pulmonates also, their transmission takes place for a longer 

 or shorter distance through the same duct, the division into male and female 

 ducts occurring later (Fig. 104 C), we may with safety assume that these two 

 ducts have arisen phylogenetically also through the splitting of one duct and 

 that thus the Opisthobranchs exhibit the more primitive condition. If we 

 then take one step further back, we may trace back the common efferent duct 

 of the hermaphrodite Gastropoda to the efferent apparatus of the dioecious 

 forms. We here naturally presuppose that we regard the separation of the 

 sexes as the primitive condition and hermaphroditism as the ^derived condi- 

 tion. Since also in dioecious animals, ova are often met with in the testis 

 and vice reran, and, further, in other divisions of the animal kingdom in which 

 separation of the sexes is the rule, hermaphroditism occurs in a few highly 

 differentiated forms, such an assumption is not inadmissible. 



The question now arises how the connection is established between 

 the penis and the genital aperture in those forms in which these two 

 arise separately. In the Opisthobranchia, a groove runs from the 

 aperture of the common duct to the introvertible penis which here 

 also is found near the right tentacle (Fig. 104 B). Where the com- 

 mon duct becomes divided up into a female and a male portion the 

 channel starts from the aperture of the latter, and, by the closure of 

 the groove and its detachment from the ectoderm, the part of the 

 vas deferens arises which lies nearer the penis, the process being 

 similar to the formation of the secondary vas deferens in the Proso- 

 branchia (Fig. 104 C). It is in any case probable that, ontogenetic- 

 ally, the formation takes place in this way, although this has not yet 

 been proved. 



In Fig. 104 A-E we have attempted to give some idea of the way in which 

 these processes may have taken place. The modifications brought about by 

 the earlier closing of the channel, by the later separation of the male duct, 

 and by the invagination, in the course of ontogeny, of the rudiment of the 

 penis (D) are self-evident. 



