Dr. G. Biitschli on the Gustrea- Theory. 375 
cells were uniformly distributed intermixed over the surface 
of the sphere. 
To this may be added further, as indeed has already been 
pointed out by Metschnikoff and others, that the advantages 
of a commencing invagination of one half into the other are 
not very intelligible; and in this view I also entirely agree. 
Perhaps, therefore, it would be better to allow the bilamellar 
form to originate from the blastula-stage referred to by a 
process of so-called delamination, therefore in the way which 
Metschnikoff and Lankester regard as the more primitive. 
But even this view seems to present very serious difficulties. 
The hypothesis put forward by Lankester seems to be quite 
inadmissible, namely that the inner ends of the cells of the 
unilamellar blastula were particularly entrusted with the assimi- 
lation of the nutriment, and finally even split themselves off 
as independent entodermal cells. It is difficult to recognize 
in this an advantage to the collective body. We might say 
that the separated entodermal cell is deprived of its better 
half, 7. e. the ectodermal part, which brought it nourishment ; 
nay, one might really say with justice, it is deposed from its 
function. But to make up for this, according to Lankester’s 
conception, a further change now occurs, that is to say, the 
reception of nourishment becomes concentrated upon one spot 
of the surface of the sphere, and the nourishment penetrates 
here into the intestinal cavity, at first without the existence of 
any mouth-opening. In the first place, this localization of 
the reception of nutriment would certainly be a disadvantage, 
and not an advantage ; and further, this profound change of the 
whole process of reception of nourishment is supposed to take 
place without any visible cause, and, moreover, quite suddenly. 
I think, theretore, that we cannot accept as satisfactory 
Lankester’s hypothesis as to the origin of the bilamellar 
embryonal form. Metschnikoff’s idea also seems to me to 
suffer under inherent improbabilities, in many respects corre- 
sponding with those which have been brought forward against 
Lankester’s hypothesis. Metschnikoff supposes that indi- 
vidual cells out of the unilamellar cell-wall of a blastuloid 
primitive form wandered into the interior of the vesicle, and, 
indeed, especially such cells as had received nourishment 
particularly abundantly. This immigration, which was 
originally only occasional, gave rise finally to a constant accu- 
mulation, a central mass of cells, ¢. e. to the formation of an 
entoderm, which originally neither enclosed a central primitive 
intestinal cavity nor was accessible through a mouth-opening. 
As, however, the formation of an entoderm is inconceivable 
