476 
the ventral border. The outer membrane of the wall of the vesicle 
is interrupted at many points, and here direct continuity exists between 
the wall and the clump of cells. I have frequently obtained cases 
where there can be no doubt that cells are in the act of migrating 
from the vesicle into this mass, which is the rudiment of the 
pericardium. Fig. 1 represents a cross section of this region of 
the vesicle after the rudiment, (pe.r.,) has attained a considerable size. 
At the points indicated by the lines marked a and 6 two nuclei are 
shown at different stages of the process of passing out of the wall, 
so that there can be no doubt as to what is taking place. Sections 
like this could be multiplied indefinitely and many figured in which 
the boundary line of the wall is absent over a considerable area. 
The pericardium is therefore derived directly from the inner vesicle, 
as RITTER (l. c.) has described for Perophora annectens. But 
around the periphery of the rudiment the cells lie loosely together, 
and there is every indication that free amoeboid cells of the blood 
are being added to the mass from the outside and aiding in its 
formation to a slight extent. This is shown more or less distinctly in 
the section drawn in Fig. 1. I might also state here that I have 
seen, especially at very early stages, clear cases of cells migrating 
from the inner vesicle into the body-space where they are set free. 
(See Fig. 6 a, and also lower right hand corner of Fig. 5.) 
This must, therefore, be one source at all events from which 
cells of the blood are derived, as Rirrer holds for Perophora 
annectens, and it is highly probable that they supplement the 
cells which are given off directly from the wall of the vesicle to the 
rudiments of the pericardium and other organs. 
The pericardial rudiment soon acquires a cavity, loses its con- 
nection with the vesicle and forms a closed sac which gives rise to 
the pericardium and heart in the usual manner. 
One of the first structures to make its appearance is the 
endostyle, which is indicated by a distinct groove running through 
almost the entire length of the inner vesicle, at a very early stage, 
namely, long before the folds which will cut off the peribranchial 
sacs, are seen. 
The latter are formed symmetrically by two folds which appear 
in the ventral wall of the vesicle on each side of the endostylar 
groove, and as they deepen, grow in to meet each other in the manner 
described for Botryllus, Perophora and other Ascidians. The 
median portion, or atrium, of the peribranchial cavity is therefore cut 
