Budding in Perophora. 217 
vesicle, except on the side where the wall is thick ; as develop- 
ment proceeds the relative difference in thickness of this 
portion and the rest of tlre vesicle becomes greater. 
While the rotation is taking place the ventral wall of the 
inner vesicle in the posterior region is folded up at the point 
indicated in fig. 2 by the line Z. PBC. As this furrow deepens 
a portion of the vesicle which is connected with the stolonic 
partition is thus gradually folded off to form the left peri- 
branchial sac. 
Ritter *, in a recent note on the budding of Perophora, says 
that “‘when the differentiation of the ‘endoderm’ into the 
branchial and two peribranchial sacs takes place, it does so in 
such a way that the developing blastozooid is connected with 
the double-walled partition of the stolon, not by the branchial 
sac, as has been hitherto supposed, but by the left peri- 
branchial sac.’ He, however, does not describe how the 
process takes place; from an examination of figs. 2 and 3 this 
can be clearly seen. ; 
I cannot confirm Ritter’s statement (page 367 of the paper 
just referred to) that the connexion between the stolonic 
partition and the left peribranchial sac is lost at an early stage, 
namely “ at a time when the two peribranchial pouches have 
merely begun to envelope the branchial sac.” I find that it 
persists until a much later time and is still present, although 
greatly constricted, at a stage when the gill-slits are about to 
be formed. 
The right peribranchial sac is formed before the rotation is 
completed at about the stage shown in fig. 2 by a longitudinal 
folding-in of the right wall of the inner vesicle. The fold 
first appears anterior to the rudiment of the pericardium and 
at about the level of the upper border of the latter structure ; 
as it deepens and extends posteriorly the portion of the inner 
vesicle thus folded off is carried down towards the ventral 
side as the rotation continues, in the same way as the peri- 
cardial rudiment. 
The section drawn in fig. 2 is taken too far back to show 
the fold of the right peribranchial sae. 
Fig. 3 represents a stage when the rotation is about com- 
pleted, and the two folds of the peribranchial sacs are now 
symmetrically placed, one on each side of the median ventral 
line. 
In this figure a collection of cells (JN 7.) is seen lying against 
the outer surface of the left peribranchial sac; these represent 
the extreme tip of the intestine where the section has passed 
* ¢ Anatomischer Anzeiger,’ x. Bd., no. 11, p. 367. 
