128 Mr. W. K. Brooks on the 



at the point where its ectoderm folds upon itself to become 

 continuous with that of the embryo. The straight stolon of 

 SaJpa pinnata is so favourable for studying its origin, and 

 the evidence that it is ectodermal is so simple and clear in 

 this species, that it does not seem necessary to devote much 

 space to the discussion of the observations which have been 

 made on twisted stolons like that of Salpa democratica, 

 where it is very difficult to study the young stages by sections. 

 The connexion between the nerve-tube and the endoderm 

 is shown only by very young stolons and for only a short 

 time, and the two structures are quite independent in older 

 stolons. 



Of the various writers on the subject, Kowalevsky (" Bei- 

 trage," &c , Nachr. d. k. Gesellsch. der Wissenseh. zu 

 Gottingen, 1868, 19) seems to regard it as mesodermal in 

 origin. Salensky, in his paper on the budding of Salpa 

 (Morph. Jalirb. 1877, iii.), says nothing about its origin. 

 Todarro (' Sopra lo svilluppo,' &c., 1875) derives it, as he 

 does all the other organs of the stolon, from a single germo- 

 blastic cell ; but I have already shown that his germoblastic 

 cell is a migratory placenta-cell, and all recent writers have 

 justly rejected his account of the stolon. Seeliger believes 

 that in the stolon of Salpa^ and also in the buds from the 

 ascidiozooids of Pyrosoma, it is mesodermal in origin, and 

 that it is derived from an indifferent mass of mesoderm, 

 which, in the young stolon, fills all the space between the 

 ectoderm and the endodermal tube, and becomes differentiated 

 into the nerve-tube and other organs of the stolon. 



I have not found at any stage anything in the straight 

 simple stolon of Salj/a pinnata corresponding to his indiffe- 

 rent mesoderm, although I have studied it in serial sections 

 in the three rectangular planes, and I do not hesitate to affirm 

 that Seeliger has been misled through the selection of a most 

 unfavourable species. 



As I have not myself studied Pyrosoma, I am not in a 

 position to make any comment on his account of this animal, 

 although Salensky (" Embryonalentwicklung der Pyrosoma," 

 Zool. Jahrb. v., 1891) has recently shown that the ganglia 

 of the four primary ascidiozooids which are produced from 

 the stolon of the cyathozooid, as well as the ganglion of the 

 cyathozooid itself, are derived from the ectoderm. 



The Ganglia of the Aggregated Salpa;. — The nerve-tube 

 arises as a solid rod^ but it soon acquires a lumen. As the 

 ectodermal folds grow inwards and mark out the bodies of the 

 Salpse they cut the tube up into a series of ganglionic vesicles, 

 one for each Salpa, with cavities which are segments of the 



