378 LEFEVRE. [Vor. XIV. 
The stolonic partition remains stationary, and the displace- 
ment or shifting around of the inner vesicle takes place on this 
as a fixed support. The process might be illustrated by the 
drooping of a flower to one side on its stem, although the 
change of position cannot be a purely passive falling over of 
the vesicle. In Fig. 9 a small collection of cells (fe.r.) is seen 
adhering to the wall of the vesicle high up on the right side, 
and these, as we shall see below, form the rudiment of the 
pericardium. This cell mass remains fixed at the same place 
on the wall, and during the shifting of the vesicle is borne 
down towards the ventral side, describing in its descent an 
arc of about 90°. It therefore furnishes a good register of the 
progress of the displacement of the vesicle. 
As the turning proceeds, the difference in thickness between 
what was at first nearly the whole right side and the rest of the 
vesicle becomes more marked; consequently, the cells compos- 
ing the entire vesicle, except in the thicker region, are seen to 
be growing more and more flattened. 
The displacement is most probably brought about by a rapid 
growth and flattening of the cells composing the greater por- 
tion of the vesicle, whereby the actual right side, which is 
morphologically the ventral side of the vesicle, is shifted or 
pushed ventrally through go°. 
This process is analogous, at all events, with the rotation or 
displacement of the pharynx of the Amphioxus larva from right 
to left, although I am not prepared to claim any phylogenetic 
relation between the two. 
By comparing Fig. 9 with Figs. 10 and 11, Pl. XXX, the 
process can be readily understood. The shifting, however, 
involves the anterior end of the vesicle only to a slight extent. 
In this region a difference in thickness of the walls is not 
observed, and the rudiment of the dorsal tube, which definitively 
has a median dorsal portion in the anterior end of the bud, 
arises as a collection of cells almost at the same time as the 
pericardial rudiment appears, lying a little to the /eft of the 
mid-dorsal line on the wall of the vesicle (Fig. 13, dt). If 
the displacement took place to as great an extent anteriorly as 
posteriorly, it is evident that this cell mass would appear much 
