336 Miscellaneous. 



to the investigations referred to, the peribranchial vesicle of the 

 larva proceeds from its ectoderm. The entire bud-rudiment of the 

 Botryllidse consequently consists, as in the case of the Bryozoa, of 

 two epithelial lamellae of ectodermal origin, ■with mesodermal cells 

 scattered in between. It follows from this that the gemmation of 

 the Botryllidte differs in this respect from that of the other Asci- 

 dians, such as Peropliora, Clavelina, Amaroucium, Didemnium, and 

 Distaplia, since in all these eases, either directly or indirectly 

 (through the epicardium and the stolonial septum), the inner vesicle 

 is formed from the endoderm. 



(2) From the inner vesicle are developed the intestinal tract, the 

 peribranchial vesicle, and the nervous system. 



The process by which the peribranchial vesicle is formed is ushered 

 in by the upheaval of the ventral wall of the inner vesicle, com- 

 mencing from in front, into two folds which project into the 

 interior. By this means from the single inner vesicle there is 

 formed a median one, which communicates posteriorly by an opening 

 on each side with two lateral ones, the peribranchial vesicles. Now 

 Delia Valle asserts that three separate vesicles arise and that the 

 two lateral ones form by fusion the single peribranchial cavity. I 

 have been unable to confirm the occurrence of this secondary fusion ; 

 on the contrary, the peribranchial cavity is formed from the 

 beginning as a saddle-shaped double vesicle, which becomes con- 

 stricted off dorsally from the median vesicle. 



The earliest rudiment of the nervous system takes the shape of 

 a nearly hemispherical evagination, situated at about the centre of 

 the dorsal wall of the median vesicle. This evagination soon grows 

 out into a tube, which is directed anteriorly and ends blindly in 

 front. The closed anterior end of the cajcum fuses afresh with the 

 median vesicle ; an aperture is formed here, and we now find upon 

 the dorsal side a tube with both ends ox^ening into the median 

 vesicle. The anterior opening lies not far from the oral aperture, 

 "which now arises as a fusion between the ectoderm and the enteron ; 

 the posterior opening of the tube communicates with the dorsal 

 portion, which connects the two lateral halves of the saddle-shaped 

 peribranchial vesicle. While the latter, as has been described, 

 becomes constricted oif from the intestine, this posterior opening 

 also closes up, and we simi)ly find a dorsal tube communicating in 

 front with the intestine. 



(3) At this stage we already observe a thickening of the ventral 

 side of the tube, which increases considerably during the subsequent 

 development. Moreover it becomes more and more constricted off 

 from the dorsal section of the tube, while delicate fibres are differen- 

 tiated in its interior. 



This thickening develops into the permanent ganglion, while the 

 tube, which gradually becomes longer and thinner, persists as the 

 hypophysis. 



I would here point out that Kowalevsky *, in the case of Didem- 



* " Ueber die Knospung der Ascidien," Archiv f. mikr. Anatomie. 



