290 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



In older embryos, Plate XXXV, the ganglion, s, and sub-neural 

 gland are clearly comparable with those of ordinary tunicates. 



SECTION 6. The Nerve Tube of the Stolon. 



The straight stolon of Salpa pinnata is so favorable for studying the 

 origin of the nerve tube, and the evidence of its ectodermal origin is so 

 simple and clear in this species, that it hardly seems worth while to 

 devote much space to the discussion of observations which have been 

 made upon twisted stolons like that of Salpa democratica, where it is 

 very difficult to study the young stolon by sections. The connection 

 between the nerve tube and the ectoderm, as I have described it on p. 70, 

 is shown for only a short time, however, in very young stolons, and 

 older stolons furnish no evidence whatever as to its origin. 



Of the various writers on the subject, Kowalevsky (Beitrage zur 

 Entwicklungsgeschichte der Tunicaten, Nachrichten der konigl. Gesell- 

 schaft der Wissenschaften zu Gottingen, 1868, 19) seems to regard it as 

 mesodermal in origin. Salensky (Ueber die Knospung der Salpen, 

 Morph. Jahrb., 1877, Bd. Ill) says nothing about its origin. Todarro 

 (Sopra lo sivelluppo e 1'anatomia delle Salpe, 1875) derives it, as he does 

 all the other organs of the stolon, from a single germoblastic cell; but I 

 have already shown that his germoblastic cell is a migrating placental 

 cell, and all recent writers have justly rejected his account of the stolon. 

 Seeliger (11 and 15) believes that in the stolon of salpa, and also in the 

 buds from the ascidiozooids of pyrosoma, it is mesodermal in its 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 endo- 

 dermal tube, and becomes differentiated into the nerve tube and the 

 other organs of the stolon. I have not found at any stage anything in 

 the straight simple stolon of Salpa pinnata corresponding to his indif- 

 ferent mesoderm, although I have studied it in serial sections at all 

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

 that Seeliger has been misled through the selection of a most unfavor- 

 able species of salpa. As I have not myself studied pyrosoma, I am not 

 in a position to make any comment upon his account of this animal, 

 although Salensky (17) has recently shown that the ganglia of the four 

 ascidiozooids which are produced from the stolon of the cyathozooid, as 

 well as the ganglion of the cyathozooid itself, are derived from the ecto- 

 derm. 



