479 
A very young stage is figured in Fig. 2 d. ¢. r., which is taken from 
a frontal section but shows the rudiment of the dorsal tube cut 
transversely, as its position on the extreme tip of the long narrow 
inner vesicle is almost at right angles to the long axis of the bud. 
This figure shows the origin from the wall of the vesicle very distinctly, 
and also the fact that cells of the blood appear to be attaching 
themselves to the young rudiment. 
The rudiment soon becomes compact. and rounded off and the 
lumen which appears in its centre is eventually put into communication 
anteriorly with the cavity of the branchial sac. 
The ganglion is differentiated out of the dorsal wall of the 
hypophysis along the entire length of the latter and finally becomes 
completely separated from it. The beginning of the rudiment is shown 
in Fig. 3 gl. r., where it is merely indicated by a slight thickening 
of the dorsal wall before there is any separation. In Fig. 4 gl, the 
ganglion has practically attained the adult structure. Between these 
extremes all intermediate stages may be clearly observed. The 
ganglion is not constricted off from the hypophysis, as Hsorr‘) has 
described for Botryllus, but after the thickening has proceeded to 
a certain extent, a new outer membrane appears between that portion 
which will form the ganglion, and the rest of the wall, thus causing 
a complete separation. 
Along the posterior half of the dorsal tube the ventral wall 
breaks down completely; nuclei multiply rapidly and wander out into 
the space between the tube and the roof of the branchial sac, where 
they are found scattered about in a mass of loosely reticular cyto- 
plasm; it is out of this tissue that the sub-neural gland is 
formed, Fig. 4 s. n. g. 
Before there is any indication of the peribranchial folds, the 
digestive tract grows out as a blind diverticulum from the posterior 
end of the inner vesicle on the dorsal side immediately in front of the 
connection with the stolon. It bends sharply to the left and growing 
forward, ultimately unites at its free end with the posterior wall of 
the atrium, into which it opens by the anus. 
The “organe réfringent” arises as a tubular diverticulum 
from the dilated region of the digestive tract, and becoming much 
branched, surrounds the long intestine with its terminal twigs. 
At quite an early stage of the development a small clump of 
cells is found lying against the wall of the inner vesicle just in front 
4) Mitth. Zool. Station Neapel, Bd. 10, 1893. 
