Origin of the Organs of Salpa. 135 
paired ectodermal invaginations from the surface of the body 
is clear and unmistakable. 
The Digestive Organs of the Embryo.—The cavity of the 
pharynx arises in the 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 
youngest embryos mistaken the two perithoracic 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 description 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 endostylice folds of the 
pharynx of the embryo. 
The pharynx 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 the 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 post-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 
