Orujin of the Organs ^/'Salpn. 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. 



IVie Ectoderm of the Stolon and that of the Aggregated 

 Salpce, — 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 (" l^'io, Knospung der Salpen," Jena. 

 Zeitschr. 1885, p. 13) that it is an evagination ('' Ausstiilp- 

 ung ") fi'om 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, so that it is folded off from 

 the body of the embryo rather than pushed out, and, in the 

 young stages of Salpa pintiata 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 Salpge has been correctly 

 held to be directly derived from the ectoderm of the stolon by 

 all students except Todavro. 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 iu 

 the segmentation of the endodermal tube and the genital rod. 

 The JServous Sgsteni 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. Tiie 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. The 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* 



