412 LEFEVRE. [Vor. XIV. 
lutely convincing to my mind, and, in fact, he himself states (oc. 
cit., p. 197) that his “conclusion rests upon a preponderance of 
evidence,’ as there is certainly evidence which is “ indicative of 
a mesenchymal origin of the cells.” I do not consider as valid 
his argument that the connection between the wall of the inner 
‘vesicle and the pericardial rudiment is too strong to be a mere 
contact, and cannot be supposed to be a secondary one, as a 
complete separation takes place later. A glance at Pl. XXXII, 
Figs. 25, a, 6, and 26, a, 6, of this paper will show that, even 
granting for the moment that the cells of these two rudiments 
have been given off from the vesicle, they are very loosely 
attached at this early stage to the wall, and that, however inti- 
mate the connection between the two may become later, it must 
necessarily be a secondary one. In P. vzridis I believe it to be 
merely a contact, although a firm one, and as the pericardial sac 
ultimately breaks away from the vesicle, on @ priori grounds 
alone one would not expect to find an organic union; I have the 
additional evidence that I have been unable to discover any 
interruption in the boundary line of the vesicle, there being 
everywhere a sharp demarcation between the wall and the rudi- 
ment. Any argument from this source must, therefore, be ruled 
out, for it is certainly true that a firm secondary contact is not 
only possible, but actually does occur in P. viridis. 
It is to be regretted that Ritter did not observe earlier 
stages in the development of the pericardium and dorsal tube, 
but his Figs. 68 and 74, and the statement on page 197 in regard 
to the pericardium, that he has found ‘no sections in which at 
some focuses | cannot see the separating line to be interrupted,” 
strongly incline one to accept his conclusions for P. aznectens. 
I share the belief with Ritter that there is very good ground 
for holding that the inner vesicle gives off cells into the body 
space, and is, therefore, one source, at all events, of mesenchyme 
cells, which are found free in the blood, and apparently take 
part in the formation of the pericardium and dorsal tube. It is 
hardly possible to suppose, as he states, that these cells are all 
derived from the mesenchyme of the embryozodid, and, in fact, 
the inner vesicle of the buds is about the only place where one 
could look for their origin. I do not consider, however, that 
