Ori(jin of the Organs o/'Salpa. 135 



] aired ectodeniial invag-inations from the surface of tlie body 

 is clear and unmistakable. 



The Digestive Organs of the Embryo. — The cavity of the 

 pharynx arises in ihe mass of visceral follicle cells by the 

 degeneration of these cells. Its endodermal epithelium is 

 derived from the blastomeres, and the gut is formed later as a 

 diverticulum from the pharynx. 



Salensky's account of the origin of the digestive organs is 

 scattered through the pages of his memoir in such a way that 

 it is hard to review ; and as I have shown that he has in the 

 } oungest embryos mistaken the two ])erithoracic tubes for the 

 pharynx, that he has at a somewhat older stage mistaken the 

 median atrium or cloaca for the pharynx, and that he has 

 failed to discover the actual pharynx during its early stages, 

 it is clear that his descrij)tion has no value. 



The Endodermal Tube of the Stolon and the Digestive Tract 

 of the Aggregated Salpa. — The endodermal tube of the stolon 

 is formed as a diverticulum from the ventral middle line of 

 the pharynx of the solitary Salpa, and its communication with 

 the cavity of the pharynx is retained throughout the whole 

 history of the stolon. The thickened side-walls of the endo- 

 dermal tube are derived from the two endostylic folds of the 

 pharynx of the embryo. 



The pliarynx of the aggregated Salpa is morphologically 

 bilaterally symmetrical with the middle plane of the stolon, 

 although this fact is obscured by the secondary changes of 

 position. It is formed from two pharyngeal pouches, a right 

 and a left, from the sides of the endodermal tube of the stolon. 

 The oral ends of these pouches grow forwards and quickly 

 meet and unite on the morphological middle line to form the 

 oral end of the pharynx. The aboral ends grow backwards 

 and approach each other on the middle line, and finally unite, 

 although they remain separate very much longer than the 

 oral ends. 



In a strict sense tlie pharynx is not actually, but only appa- 

 rently double in origin, for the middle section or endodermal 

 tube is not double. Fundamentally it is a single unpaired 

 expansion of the endodermal tube, but at the time when it 

 elongates towards the oral and aboral ends of the body the 

 middle line is occupied by the blood-tubes, and it pushes along 

 the sides of these structures, and does not become complete 

 in the middle line until a much later stage. 



The jjost-pharyngeal gut arises as a blind diverticulum 

 from the aboral end of the right pharyngeal pouch. The 

 part of the diverticulum nearest the pharynx becomes the 



