. BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 63 



entiated cells, which will go to make up the testis, are characteristic of 

 all the younger buds. On the other hand, the amount of separation of 

 the ovotestis from the inner vesicle is subject to considerable variation. 

 In the bud figured, the two apparently are fused in places, but in other 

 buds, which are entirely similar in all other respects, the line of demar- 

 cation of the ovotestis from the inner vesicle is most distinct. In these 

 cases it is seen that all the mesodermic elements on the dorsal side of 

 the inner vesicle, with the exception of a few scattered cells, are con- 

 tained in the fundament of the ovotestis. The conclusion, then, to be 

 drawn from the examination of the undifferentiated bud is that the 

 possibility of an endodermic origin for some of the unspecialized cells 

 of the ovotesticular fundament must be conceded ; but certainly the 

 greater part, and perhaps the whole of it, is derived from the mesoderm. 



So far as the spaces between the cells of the ovotestis are concerned , 

 the figure represents a rather unusual condition, since in most cases the 

 fundament forms a compact mass. But on account of the unusual sepa- 

 ration, this bud demonstrates very clearly that in this stage Distaplia 

 has no " assise peripherique " such as Julin ('93, p. 97) and Floderus 

 ('96, p. 175) found in the youngest stages examined. 



In slightly older buds, where the fundaments of the intestine and 

 epicardium are present, the only change in the ovotestis is due to a few 

 of the ova having increased in size and assumed more distinctly the 

 characters of the oogonium. Its position is still dorsal, but it occupies 

 only the posterior end of the dorsal side of the bud. At still later 

 stages, even after the peribranchial sacs are quite large, the ovotestis 

 is apparently fused with the posterior end of the dorsal tube. This 

 connection is often very puzzling; in one series, for instance, the 

 lumen of the dorsal tube extends quite to the ovotestis, the latter 

 appearing as a thickening of the dorsal tube. In buds of these stages 

 there is considerable variation in the disposition of the organs, the 

 ovotestis being placed slightly to one side of the dorsal tube or directly 

 behind it, or even dorsal to its posterior end. Accordingly, I am 

 inclined to believe that the apparent union between these two organs 

 is not significant, but due rather to their close approximation and to 

 oblique sectioning. This position is further strengthened by the fact 

 that both before, immediately after, and even occasionally during, 

 this stage, the ovotestis is completely isolated from all other organs. 

 Whenever the ovotestis is thus isolated, it is produced anteriorly into a 

 genital strand, which occupies very nearly the mid-dorsal line. It 

 seems likely, then, in view of the very general isolation of the sexual 



