220 Mr. G. Lefevre on 
dorsal mid-line. By this time a lumen has appeared in the 
centre, and around this the cells gradually arrange themselves 
into an epithelium to form the dorsal tube (fig. 4, C). The 
Fig. 4. 
latter fuses later at its anterior extremity with the dorsal wall 
of the pharynx, and an opening breaks through; at no time 
is there any communication posteriorly between the tube and 
the peribranchial cavity. 
The nerve-ganglion is formed by a thickening of the dorsal 
wall of the tube, which eventually becomes constricted off in 
the manner described by Hjort for Botryllus, although in the 
latter it is the ventral wall of the tube which gives rise to the 
ganglion. 
Summary. 
1. By a peculiar process of rotation of the endodermal 
vesicle through 90° the thickened right wall of the vesicle is 
carried down to the ventral side of the bud-rudiment, where 
it forms the floor of the future pharynx. This process seems 
to be due to rapid growth and flattening of the cells com- 
posing the vesicle except in the thickened portion. 
2. The pericardial rudiment, which is the first organ to 
appear, is formed from the free cells of the blood. It arises 
on the right side of the inner vesicle, and through the rota- 
tion of the latter is brought down to the ventral side. 
3. The peribranchial sacs arise asymmetrically. As the 
rotation takes place, the ventral wall of the inner vesicle is 
folded up immediately to the right of the point where the 
stolonic partition joins the vesicle, to form the left peri- 
branchial sac. ‘The portion of the vesicle thus folded off is 
continuous with the stolonic partition; the connexion with 
the latter persists until quite a late stage. A fold appearing 
on the right side of the vesicle gives rise to the right sac, 
which then moves ventrally as the rotation continues. 
4, No epicardium is present; in this respect Perophora 
differs strikingly from Clavelina and some other Ascidians. 
5. The endostyle appears early as a longitudinal groove in 
