481 
free cells of the blood are also involved to some extent in their 
formation. In P. viridis, however, I believe that these organs are 
derived entirely from the latter source. A detailed paper on the 
budding of this Ascidian will shortly appear, and in it I have presented 
the full evidence for this belief. It is certainly true that cells do 
not pass out directly from the vesicle into the rudiments of the above 
mentioned structures, but although I have observed no cases of cells 
being given off to the blood at other points on the wall, it is possible 
that in some buds or at certain times at least such an occurence does 
take place. This supposition is made probable by the fact that cells 
of the blood are known to arise from the vesicle in Perophora 
annectens and Ecteinascidia. In these latter forms, therefore, 
the organs in question are derived in large measure directly from 
the primitive vesicle, while in Perophora viridis their origin from 
the vesicle would be traced only indirectly through cells of the blood, 
if the latter, as is probably the case, come from this source. At all 
events the two processes would not be essentially different in nature, 
the inner vesicle being the starting point in both cases. 
The primitive vesicle is therefore the all-important part of the 
bud-rudiment and upon it devolves the duty of providing material for 
the formation of all the internal structures, as the ectoderm plays no 
active role in the development except in so far as it is concerned in 
the production of the cellulose test. ‘This conclusion apparently holds 
good for all Ascidians whose bud-development is known, with the 
possible exception of Perophora viridis, and has been sufficiently 
proved to be accepted as a general rule. 
The inner vesicle alone, whether derived from an ectodermal or an 
endodermal larval structure, gives rise to all the corresponding organs 
which in embryonic development proceed from the two primary 
germ-layers, and in some cases also furnishes cells to the blood, 
e. g. in Ecteinascidia and Perophora annectens. It is 
therefore an undifferentiated structure, and like the blastula, contains 
within its cells the potentiality of the whole organism. 
All parts of the simple epithelial wall, however, are not alike, 
but one definite area furnishes the material out of which one organ 
is built up, while another clearly defined region produces a second 
structure, and so forth. 
But this internal differentiation or mapping out of the vesicle, is 
present in different degrees in different Ascidians, and in fact we are 
able to pick out a series of forms which present a gradation from the 
