W. K. BROOKS ON THE GENUS SALPA. 299 



SECTION 10. The Digestive Organs of the Embryo. 



I have already, p. 31, described the way the cavity of the pharynx 

 arises in the mass of visceral follicle cells by the degeneration of these 

 cells. I have shown also that its endodermal epithelium is derived 

 from the blastomeres, and, page 33, that the gut is formed as a diverti- 

 culum from the pharynx. 



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

 through the pages of his memoir (5) in such a way that it is hard to 

 review, and as I have already shown, page 293, that he has in the younger 

 embryo mistaken the two perithoracic tubes for the pharynx, and 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 stage, it is clear that his description has no value. 



SECTION 11. The Endodermal Tube of the Stolon and tlie Digestive Tract 



of the Aggregated Salpa. 



On page 71 I have traced the origin of the endodermal tube of the 

 stolon. My own observations on the origin of the pharynx by the union, 

 on the middle line, of the two pharyngeal diverticula from the endo- 

 dermal tube of the stolon have already been described, and as I have, 

 page 95, outlined their complicated history to the best of my ability, 

 there is little to add. I must repeat, however, that the pharynx is not 

 actually, but only apparently, double in origin. While its oral and 

 aboral ends consist, for a long time, of pouches which are separated 

 from each other on the middle line, the middle section d' is never double. 

 Fundamentally the pharynx is a single, unpaired expansion of the endo- 

 dermal tube of the stolon, as it is in pyrosoma, and as I have repre- 

 sented it in the diagram on page 79; but at the time when it elongates 

 towards the oral and aboral ends of the body, Plate V, Figs. 1 and 2, the 

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

 along the sides of these structures, and does not become complete on the 

 middle line until a much later stage. 



The oral ends of the two pouches, Plate V, Fig. 4, unite very much 

 sooner than the aboral ends, and in Salpa cylindrica these latter remain 

 separated from each other, even after the body is fully formed, by a 

 median ventral mesentery, Plate XXXIX, Fig. 11, which separates the 

 right half of the pharynx, 27, from the left half, 28. 



