THE FORMATION OF THE ORGANS THE GENITAL ORGANS. 221 



rudiments. .The whole apparatus, the germ-gland included, has 

 been derived from a single ectodermal thickening which extends and 

 becomes differentiated later (RouzAUD). There can, however, be 

 but little hesitation in at once excluding this view, inasmuch as the 

 hermaphrodite gland is, in any case, yielded by the mesoderm. [The 

 gonad is probably derived from the apparently undifferentiated blasto- 

 ineres.] With regard to the ectodermal part, i.e., the efferent ducts 

 and accessory structures, there may certainly be two distinct types 

 of development according as the copulatory organ lies separately 

 from the female genital aperture or is united with it in a common 

 atrium. In the first case, the genital apparatus would, as in the 

 Prosobranchs (Palndina), consist of three parts, riz., of the germ- 

 gland, of the primary (nephridial), and of the secondary (ectodermal) 

 efferent ducts together with the penis. This is evidently the case in 

 Liiniiara, as we may conclude from the observations of EISIG and 

 KLOTZ. 



The genital apparatus of Limnctea appears as a rudiment even 

 before the hatching of the young animal. The penis appears first as 

 an ectodermal invagination at the 

 base of the right tentacle. The 

 hermaphrodite duct arises inde- 

 pendently of it as a strand-like 

 structure. The mesodermal 

 character which has repeatedly 

 been assumed for it appears doubt- 

 fid. We are inclined rather to 

 consider it as ectodermal, a view 

 which is supported in the literature 

 on the subject. This strand is of 

 special importance, as it splits 

 later into two parts, one of which 

 represents the rudiment of the 

 uterus and the other that part of 

 the vas deferens w r hich is known 

 as the prostate (Fig. 103, ut, vd). 

 The hermaphrodite gland arises 

 independently of this strand from 

 the mesoderm [? primitive blastomeres] . A short process of the 

 mesodermal rudiment yields the proximal portion of the efferent 

 duct, while the distal part arises from the cellular strand mentioned 

 above (Fig. 103, &/, BROCK, KLOTZ). The spermetheca becomes 



I -' 



..all. 



FIG. 103. Diagram representing a later 

 stage in the development of the genital 

 organs of a Pulmonate. alb, albumen - 

 gland ; p, penis ; rs, spermetheca ; ut, 

 uterus ; vd, vas deferens ; zd, hermaph- 

 rodite gland ; zg, hermaphrodite duct ; 

 5 and . genital apertures. 



