374 Dr. G. Biitschli on the Gastrwa- Theory. 
received a sort of confirmation by a discovery which came 
from quite another side. 
When the business is to ascertain, in a speculative fashion, 
the course of a phylogenetic process, it seems to me that in 
general a discussion of the primitiveness of the ontogenetic 
process is very difficult, and has little prospect of result. It 
appears to me, on the other hand, to be much more important 
that for certain stages of the assumed phylogenetic course of 
development there are at the present day, or have been for- 
merly, representatives which demonstrate the possibility of 
the existence of these stages. Finally, it seems to me very 
important that the changes of the assumed forms should be 
readily intelligible, and occur gradually, not suddenly, and 
also should be really advantageous. In the last respect espe- 
cially I think that the new view now to be developed possesses 
some advantages over its predecessors. 
The starting-point of my observations was formed by the 
colonies of the Flagellata, for, as has already frequently been 
urged by myself and others, we must undoubtedly connect 
the derivation of the Metazoa with some such forms. I[ 
believe that in connexion with this, moreover, it is of com- 
paratively little consequence whether the Flagellata that 
we adduce for comparison possess either a more animal or 
a more vegetable mode of nutrition, as the physiology of 
nutrition varies much, without reference to the morphology, 
in the section Flagellata. Now we certainly find among the 
colonies of the Flagellata not a few which in their structure 
represent a so-called blastula-form, as among the Volvocinese 
the genera Volvox and Hudorina, and, further, especially the 
genus Uroglena, and approximately some others. 
Nevertheless the difficulty of deriving a bilamellar form 
from such colonies appears to me to be very considerable, and 
so, indeed, according to either of the hypotheses previously 
mentioned. ‘The assumption of the invagination of such a 
blastula-stage presupposes the differentiation of one half, or, at 
any rate, of a section of the cell-sphere. ‘The one half would 
become nutritory, and therefore the entoderm, while the 
other would remain essentially locomotory. Liven this differ- 
entiation will be difficult to bring into harmony with the 
presupposed spherical formation. Such a cell-vesicle, in 
connexion with its general uniformly rotating movement, 
will present little chance of the occurrence of a differentiation 
into two different half-spheres. Should a differentiation of 
two kinds of cells occur, it would certainly be much more 
advantageous to such an organism if the different kinds of 
