BUDDING IN TUNIC AT A. 55 



the secondary ascidiozooids of Pyrosoma (29), derives the 

 nervous system from the mesoblast ; and attributes a 

 common origin to the nervous system and genital cells. 

 Such an origin, if true, would be extremely remarkable ; it 

 may be compared with the recent observations of Korotneff, 

 who states (12) that in the buds of Dolchinia also the nervous 

 system and genital cells are derived from a common 

 rudiment, although in this case the common rudiment is 

 imagined to be endodermal. It must be borne in mind, 

 however, that the nature which Korotneff attributes to the 

 three primary elements which he describes in the buds of 

 DolcJiima is purely hypothetical. And, so far as Seeliger's 

 statements are concerned, it is in the highest degree 

 probable that the ectodermal origin of the nervous system 

 which has been recently demonstrated in the case of Salpa 

 by Brooks (4) and Korotneff (14), and in Distapiia and the 

 primary ascidiozooids of Pyrosoma by Salensky (24, 25), will 

 eventually be extended to the secondary buds of Pyrosoma. 

 It is also very probable that Kowalevsky's account of the 

 development of the nervous system in Distapiia stylifera is 

 not altogether accurate. In the ventral buds of a species of 

 the same genus Salensky has recently shown that the nervous 

 system develops from a pair of large ectoderm cells, which 

 are differentiated very early in the formation of the stolon, 

 and then sink beneath the general ectoderm. If, as is 

 probable, a similar mode of development also occurs in the 

 posterior buds, it is doubtful whether the methods employed 

 by Kowalevsky would have enabled him to recognise it ; 

 and I am inclined to think that he has mistaken for the 

 dorsal nerve-cord an associated organ which will be referred 

 to below as the dorsal tube, and which is undoubtedly of 

 endodermal origin. The observations of Van Beneden 

 and Julin upon the ectodermal origin of the nervous system 

 in the buds of Clavelina, a form which is nearly allied to 

 Distapiia, support this idea ; but, as pointed out by Pizon, 

 the stages described by these investigators were not 

 sufficiently complete to justify us in accepting their results 

 upon this point until further evidence has been obtained. 

 We now come to Botryllus, an exceedingly interesting 



