408 LEFEVRE. [Vou. XIV. 
doubt, as it would, of course, be impossible to prove that all or 
any of them are direct descendants of the embryonic mesoderm, 
and the most diligent search fails to show that the inner vesicle 
of the bud is the source from which they are derived. Even 
though it cannot be shown to be the case, however, it is quite 
possible that at certain times the primitive vesicle does give 
off cells which are set free in the blood, and, as this is 
an undifferentiated structure and plays such an important réle 
in the development, the supposition is probable. But, what- 
ever be their origin— whether it is the mesoderm of the 
embryo or the inner vesicle of the bud-rudiment or both — 
amoeboid cells floating freely about in the blood spaces become 
aggregated at certain places and form pericardium, dorsal tube, 
ganglion, and sexual organs. 
A discussion of the nature of these cells and the part played 
by them in the developing bud will, however, not be out of 
place. In the case of a fixed organ, like the ganglion or peri- 
cardium, which has a definite and determined position, the cells 
destined to form it must, by virtue of their motile power, come 
together at the right time and place. Weismann (35; pp. 161, 
162), in discussing the process of gemmation in Clavelina as 
described by Seeliger (29), supposes that “these cells contain 
very different kinds of idioplasm; one, for instance, might con- 
tain ‘muscle determinants,’ and another, ‘nerve determinants,’ 
and a third, ‘blood corpuscle determinants.’”’ He further adds 
that “until we know more of the actual facts concerned, we 
can only — however unsatisfactory such an assumption may be 
— attribute to the cells a tendency to become attached at 
definite points according to the manner in which they have 
previously been determined.” 
It seems to me, however, that the opposite assumption, 
which Weismann regards as less likely, namely, “that these 
cells develop into muscle, nerve, or sexual cells, according to 
their point of attachment,” is more in accordance with the facts 
presented by the bud development of Pevophora viridis. 
We have already seen in the very young bud, when it con- 
sists merely of two simple layers, and before there is the 
slightest indication of the appearance of any organs, that cells 
