KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 63. N:0 9. 19 



cup-shaped structure here described: »Eine geringe Zahl von Embryonen, meist 2 an 

 einer Seite, liegen dorsal von den Hoden in den Peribranchialräumen, eingeschlossen in je 

 ein kugeliges Follikel, das mit einem kurzen Stiel an der Leibeswand, dem urspriinchlichen 

 Ort der Ovarien, sitzt» (1. c. p. 111). The meaning of the expression »ein kugeliges 

 Follikel» is not clear; it seems uncertain whether it is to be taken in the sense of 

 follicula externa, the outer envelope of the ovum, or in the sense used by Della Valle 

 and Pizon, designating the globular sac, which, according to the latter authors, en- 

 closes the embryo. But, whatever the interpretation may be, Michaelsen's state- 

 ment does not coincide with the result of the present investigation as is evident from 

 the above. As to the number of the embryos, it seems, judging from the material 

 at my disposal, as though it more often exceeded the number tvvo than fell below it. 



The epithelium forming the cup-shaped fold does not show any special dif- 

 ferentiation, nor are either the inner follicle enclosing the embryo, or the test cells 

 observed within it, differentiated in any special way. The cup-shaped folds may be 

 called placenta-like, being formed on the inner side of the peribranchial cavity by its 

 outer epithelium, but nothing in their structure has been observed that points to the 

 function of a true placenta. The mesoderm projects between the folds, and the cells 

 in the mesodermal layer are densely crowded around their base, but they do not differ 

 from the rest. A special investigation on this point, however, would be of great 

 interest. It seems to me that the cup-shaped folds are most probably to be com- 

 pared withbrood-pouches, developed to attach to themselves and to protect,the embryos. 

 It is uncertain whether the enclosing of the brood-pouches is in general to some 

 degree favourable as regards their nourishment, but such a possibility is not altoge- 

 ther excluded. At all events, as far as one can judge from the material examined, 

 the cup-shaped folds in Botryllus schlosseri do not show the complicated histological 

 structure which, according to Salensky^ 1 description, characterizes »the placenta» 

 that occurs in the cloaca of some members of the family Synoicidce, viz. Fragarium 

 areolatum, Circinalium concrescens and Amaroecium roseum. lf a renewed examination 

 should confirm Salensky's result, the presence of a true placenta in the Ascidians 

 will hardly admit of denial. Salensky distinguishes both a placenta materna and a 

 foetal placenta with a rather peculiar differentiation. The embryos of the above- 

 mentioned species are enclosed in folds formed by the wall of the cloaca. Even if 

 further investigations should confirm the different development of the structures in 

 question, I may nevertheless here call attention to the formation of cup-shaped folds 

 for attaching the embryos as an interesting point of agreement in these two groups 

 of Ascidians that are from a systematic point of view so remote from each other. 



Stage VI (Pl. I, fig. 12). The zooid figured measures about 2 mm. in length. 

 On examining it one is struck by the reduced size of the testis; when it is compared 

 with the corresponding organ in stage IV, the difference is seen to be striking. The 

 two embryos visible on the left side of the zooid are in rather an interesting stage. 

 They are already fully developed larvae and are just about to pass into the outer 



1 Salensky, W., Ueber die Thätigkeit der Kalymmocyten (Testazellen) bei der Entwicklung einiger Svn- 

 ascidien. Festschrift fur Leuckart. 1892. 



