198 RITTER. [Vou. XII. 
in many sections, and less distinctly in many others, I am sure 
that the line is interrupted. 
And, as already said, the fusion can hardly be regarded as 
secondary, because at a little later time the pericardium is 
clearly wholly distinct from the “endoderm.” Now what is the 
explanation of the facts that have left traces of doubt in my 
mind, and have led Lefevre to believe that the cells under con- 
sideration come from the mesenchyme ? I believe it to be this: 
That the mesenchyme cells themselves are being constantly pro- 
duced from various parts of the endodermic vesicle for a consider- 
able time during the early stages in the development of the bud. 
My evidence for this is not as conclusive as we would wish 
it to be, but at the same time I have observed a considerable 
number of cases in which I believe cells are being set loose 
from the “endoderm,” and are passing into the blood. Figs. 
77 and 78, Pl. XVII, represent cases of this kind, d.c.', being 
the cells referred to. These are both remote from the position 
at which either the ganglion or heart will form. 
And, on theoretical grounds, is not such an origin of the 
mesenchyme cells highly probable? Certainly new ones must 
be forming rapidly somewhere, for the newly developing zooids 
must make large demands on them for their blood and muscles, 
both of which undoubtedly come immediately from the mesen- 
chyme. To suppose that all the muscle, blood, and mesenchyme 
cells of a colony have been derived directly from the mesen- 
chyme cells of the embryozooid does, it seems to me, rather 
more violence to general developmental principles than to 
suppose that each new bud is capable of producing such cells 
for itself. And what part of the bud is more likely to do this 
than the “endoderm”? If my belief on this point is correct, 
then it is not at all impossible that the heart, or even the 
ganglion, may be formed, at least in part, from mesenchyme 
cells, for the mesenchyme cells would themselves have the 
same source, and when first severed from the ‘endoderm”’ 
would be of the same character as the cells which certainly 
produce the heart and ganglion in the buds of some other 
species. It would be merely a question of what position on 
the primitive inner vesicle the cells should be given off. And 
