Origin of the Organs of Salpa. 127 
derived from the oviduct, and that in all other species it is 
derived from the epithelial capsule (“ Epithelhiigel,” “ Ecto- 
dermkeim’’) ; but I think all will agree that his position is 
untenable until he has traced the history of these extra- 
follicular blastomeres and has proved that they take no part 
in its formation. 
I have shown that they do give rise to the ectoderm and 
that the epithelial capsule is a transitory structure which is 
lost as the ectoderm replaces it. 
The Ectoderm of the Stolon and that of the Aggregated 
Salpe.—All agree that the ectoderm of the stolon is derived 
directly from the ectoderm of the embryo. In one minor point 
my observations show that the older accounts are incorrect. 
It is usually stated that the ectoderm of the stolon is pushed 
‘ out into a tube by the growth of the other constituents of the 
stolon, and Seeliger says (“ Die Knospung der Salpen,” Jena. 
Zeitschr. 1885, p. 13) that it is an evagination (“‘ Ausstiilp- 
ung ”’) from the ectodermal epithelium of the embryo. This 
is not literally true, for the ectoderm itself takes a most active 
part in the formation of the stolon. ‘This is marked off on 
the body of the embryo by a fold of ectoderm, which pushes 
backwards from its tip to its base, sc that it is folded off from 
the body of the embryo rather than pushed out, and, in the 
young stages of Salpa pinnata at least, its differentiation from 
the body of the embryo is chiefly due to the active growth of 
this ectodermal fold. 
The ectoderm of the aggregated Salpz has been correctly 
held to be directly derived from the ectoderm of the stolon by 
all students except Todarro. My own observations show also 
that the multiplication of the ectoderm cells is the chief agent 
in the segmentation of the stolon, that the nerve-tube and the 
perithoracic tubes are cut up into vesicles by the growth of 
the ectodermal folds, and that these are the chief agents in 
the segmentation of the endodermal tube and the genital rod. 
The Nervous System of the Embryo.—Little can be said of 
this without illustrations. The caudal nervous system is 
represented by scattered blastomeres, which soon degenerate 
and disappear. The ganglion is formed as an invaginated 
fold of the somatic layer of the follicle, and the ganglionic 
blastomeres pass into it from the ectodermal ridge and become 
completely folded in among the follicle cells. ‘he ganglionic 
rudiment soon loses its connexion with the somatic layer and 
unites with the visceral follicle cells in the region of the roof 
of the anterior end of the pharynx. 
The Nerve-Tube of the Stolon.—This is formed from the 
ectoderm on the middle line of the upper surface of the stolon 
10 
