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 
Salpa 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- 
trige,” &c, Nachr. d. k. Gesellsch. der Wissensch. zu 
Gottingen, 1868, 19) seems to regard it as mesodermal in 
origin. Salensky, in his paper on the budding of Salpa 
(Morph. Jahrb. 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 Salpa 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 ina 
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 Salpe.—The nerve-tube 
arises asa solid rod, but it soon acquires a lumen. As the 
ectodermal folds grow inwards and mark out the bodies of the 
Salpz they cut the tube up intoa series of ganglionic vesicles, 
one for each Salpa, with cavities which are segments of the 
