BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 67 



ingly, there is no genital strand at all in Styelopsis, and the separation 

 of ovary from testis takes place from the middle towards both ends ; in 

 the other respects mentioned, however, this genus agrees with Pero- 

 phora, Phallusia, Clavelina, and Ciona. 



With respect to the presence of a peripheral epithelium surrounding 

 the developing ovotestis, there is more variation. In Perophora and 

 Distaplia this is present in the testis but not in the ovary, while in 

 Clavelina, according to the figures of van Beneden et Julin ('85, Planche 

 XV., Figs. 9, 10, and 13), in Styelopsis and in Ciona it is encountered 

 in both. This distribution is very suggestive, for the first two genera 

 have rather small zooids, and a simple structure, while the last three 

 are much larger, produce many more sex cells, and are more highly 

 specialized. It seems, then, that it is only in the larger and more 

 complex ascidians that this peripheral epithelium is formed around the 

 developing ovary. 



For the explanation of the fact that the oogonia are present in the 

 undifferentiated Distaplia bud, we must undoubtedly look to the fact 

 that in this genus, as in Botryllus, the life of the individual zooid is 

 very short, so that apparently it would not have time to mature an 

 ovum if the latter were to start from a wholly undifferentiated cell, and 

 consequently it receives the ovum in a slightly elaborated form. In 

 these two genera, however, the structure and development of the sexual 

 organs seem to be entirely different, as according to Pizon ('92-'93, 

 pp. 257-271), the ovary and testis of Botryllus arise from separate fun- 

 daments, and no real ovary is formed at all, but merely a few follicles 

 attached by separate stalks to the peribranchial sacs. 



Distaplia also agrees with Botryllus and disagrees with all other 

 species in which the matter has been studied in that the funda- 

 ments of the sexual organs and their ducts are usually solid. This 

 condition is probably the direct result of the early development of 

 these organs necessitating their being blocked out in solid masses 

 before enough cells to form vesicles were available. 



On the whole, however, it is evident that the development of the 

 sexual organs in our genus conforms to the type first described by van 

 Beneden et Julin for Perophora, Clavelina, and Phallusia, and deviates 

 rom it only in minor respects due to the simpler structure and short 

 flife of the zooids in Distaplia. 



