Dr. G. Biitschli on the Gastrea- Theory. 379 
supposition that a very early issue of the embryo from the 
ege-envelopes has been the rule, which, considering the 
process of development of the simplest living Metazoa, 
appears not very improbable. On the other hand, an advan- 
tage might be found in the circumstance that a blastuliform 
metamorphosis of the plate is favourable to any eventual 
nourishment of the embryo by fluid aliment introduced from 
without, the receptive surface being increased by the inflation 
into the spherical form. That such a process does occur may 
perhaps be indicated by the accumulation of fluid which so 
frequently leads to the formation of a segmentation-cavity, 
and which, at any rate, points to an imbibition of the cells 
from without. 
I might here bring forward another point, namely, that the 
developmental history of the blastula, which we see perma- 
nently preserved in certain Volvocinex, is by no means the 
same as that which we observe in the so-called invagination- 
blastula. Thus the blastula of the Volvocinez does not origi- 
nate by the central separation of the cells of a cell-aggregate, 
but by the gradual incurvation of a unilamellar cell-plate, 
during which, therefore, the cavity of the blastula, which only 
becomes closed by degrees, remains open by a sort of blasto- 
pore until the last moment of the incurvation. 
I now come to say a few words of the delamination- 
gastrula, which, as already shown, must also, in accordance 
with our hypothesis, not be an original form. As a matter 
of course the original process which produces the bilamel- 
larity of our placula is a process of delamination, and we have 
therefore at any rate to regard the delaminative production of 
the entoderm as original, and we may do this the more be- 
cause the invaginated entoderm of the invagination-gastrula 
originally also separated itself from the ectodermal cells by a 
division (delamination). Whether this separation of the 
elements of the ectoderm and entoderm takes place earlier or 
later seems to be a matter of indifference, as indeed Ray 
Lankester has already sufficiently pointed out, as it is easy to 
understand how a separation of the two kinds of elements 
would occur constantly earlier and earlier, until finally even 
the first segmentation permanently separated the ectodermal 
and entodermal elements, and indeed we see this actually 
carried out in the interesting example of Lhabdonema 
(according to Gétte). While in this way an acceleration 
of the separation of the two kinds of elements proves 
to be favourable, it certainly appears conceivable also that 
under certain circumstances a retardation might occur, 
aud this might satisfactorily explain the phenomenon of 
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