Budding in Perophora. 215 
At this stage there is found a collection of cells applied 
closely to the outer surface of the endodermal vesicle high up 
on the right side. These cells (fig. 1, PC.) appear when the 
vesicle is still spherical, and form a somewhat elongated mass 
lying in the posterior portion of the bud; they give rise 
later to the pericardium, which is therefore the first organ to 
make its appearance. At first the rudiment consists merely 
of a single layer of cells joined loosely end to end, but it soon 
becomes thicker and more compact. 
Fig. 1. 
That the pericardium is formed by the coming together of 
Sree mesenchyme-cells I believe there can be scarcely a doubt. 
At the stage represented in fig. 1 the similarity between the 
cells scattered freely about in the space between the two 
vesicles and those which form the rudiment of the pericardium 
is perfectly apparent. And, moreover, I cannot find the 
slightest evidence that proliferation of the wall of the endo- 
dermal vesicle occurs at this place; the line of demarcation 
between the two structures is distinct throughout, and shows 
no interruption in its continuity. 
A peculiar change of position now takes place in the inner 
vesicle. By a process of rotation of this vesicle through 90° 
the thickened right wall is carried down gradually until it 
comes to hie along the ventral side—that is, the side next the 
stolon, where it will later form the floor of the pharynx. The 
