52 SCIENCE PROGRESS. 



cavities are not of ectodermal origin. It would be unwise, 

 however, to attach too much weight at present to inferences 

 drawn from the embryology of Salpa, since extensive * 

 secondary changes render the interpretation of its develop- 

 ment a work of unusual difficulty. But no objection of this 

 kind can be urged with regard to the embryology of 

 Botrylhis. In this peculiar, and, as I think, primitive type 

 of Ascidians, Pizon (20) showed a couple of years ago that the 

 cloaca and peribranchial chambers result entirely from the 

 transformation of a pair of diverticula from the endodermal 

 vesicle (or primitive pharynx) of the larva. The only 

 ectodermal element is provided by the lips of the cloacal 

 aperture. Pizon's observations have not hitherto been 

 confirmed, but from the researches on this interesting form 

 upon which I am myself engaged, I am inclined to attach 

 full value to his principal results. It certainly seems 

 unjustifiable on the part of Hjort (10) to decline to believe 

 Pizon's statements on the sole ground that in Phallusia and 

 Clavelina the development of the peribranchial chambers 

 takes place differently. 



Thus, apart from general considerations, there is some 

 direct evidence that the peribranchial sacs in Tunicate 

 embryos are occasionally endodermal in origin ; and this 

 lends corresponding support to the views which Fol, Van 

 Beneden and Julin have expressed as to the homologies 

 and nature of these structures. It is true that there is no. 

 clearly established case, either among buds or larvae, in 

 which the peribranchial sacs have been shown to arise from 

 both elements ; but a little consideration will show that this 

 is no insuperable objection to the theory of their primitively 

 double origin. For when the primitive pair of gill-slits lost 

 their cilia (as, for example, has undoubtedly occurred in the 

 Appendicularian Kowalevskia), and dilated to assume the 

 functions of mere channels for the currents of water passing 

 through the newly-formed pharyngeal stigmata, there is 

 every reason to expect that the circuitous mode of develop- 

 ment — by the participation of both layers — which was 

 inherited in their earlier phases, would be replaced by a 

 more direct and simpler process. One of the two elements 



