376 Dr. G. Biitschli on the Gastrea- Theory. 
unless real advantages in nutrition thereby occur, with which, 
indeed, the differentiation of this germinal layer seems to be 
causally connected, the question is, Can we really demonstrate 
actual advantages for the process of nutrition, which render 
the immigration of the entodermal cells admissible? It seems 
to me, however, that the immigration of the entodermal cells, 
to which the reception of nourishment-is confided, cannot be 
regarded as an advantage. Without the simultaneous for- 
mation of a mouth-opening, which, as in Lankester’s hypo- 
thesis, is unintelligible and destitute of motive, the immigration 
of the entodermal cells would, in my opinion, have been only 
disadvantageous, because, if we may so speak, they would 
thereby have bolted themselves in. 
These and similar considerations, but especially the endea- 
vour to establish a plausible connexion between invagination 
and delamination, led me to the idea that probably the 
spherical blastula might not have been after all the starting 
form of the first bilamellar stages; and I believe that both 
ontogenetically and among fully-developed organisms stages 
are presented which form more satisfactory starting-points 
than the blastula for the bilamellar stage. 
Among the colonies of the Flagellata there is a genus of 
Volvocinee which is constructed upon the type of the uni- 
lamellar cell-plate, namely Gontum, and a very nearly allied 
one which occurs in the form of a unilamellar ring. When 
once the notion had occurred to me that bilamellarity might 
well have commenced in the stage of such a cell-plate, the 
conception seemed to me to offer the most favourable condi- 
tions for arriving at a satisfactory idea of the phylogenesis of 
the gastrula formation. 
Accordingly it appears to me to be assumable that the bi- 
lamellar stage first occurred in a Protozoan colony, the cells 
of which were arranged side by side in the same plane, so as 
to form a one-layered plate. ‘Then, all the cells dividing 
parallel to the surtaces of the plate, there was next produced 
a two-layered plate, of which the two layers of cells perhaps 
still showed no differentiation. or the sake of intelligibility, 
and because this has since been kept similar, we shall give 
his stage of the bilamellar plate the name of placula. 
Moreover, it is easily conceivable that the two sides of a 
unilameliar plate might develop different functions—that the 
one might adapt itself chiefly for locomotion and the other for 
nutrition, and that, finally, in the passage into the two-layered 
state, these two functions may have been localized upon the 
two cell-layers *. 
* Many reasons derivable from the structure of the Flagellata and their 
4 
