378 Dr. G. Biitschli on the Gastrea- Theory. 
Phoronis and Ascidia mentula, according to Metschnikoff. Of 
course we here leave out of consideration all those cases in 
which the original conditions appear to be obscured by an 
abundant development of nutritive vitellus. 
Our conception, therefore, requires that, in opposition to the 
ordinary pre-existing notions, we should regard the so-called 
blastula-stages not as palingenetic developmental forms. In 
this we partly agree with Lankester and Metschnikoff, both 
of whom regard the so-called invagination-blastula, 7. e. the 
blastula which becomes converted into the bilamellar gastrula 
by invagination, as a ccenogenetic form. 
My conception, however, also brings the so-called delami- 
nation-gastrula into the series of coenogenetic forms; and this 
consequence, in general, need not be regarded as contradictory, 
although the important differences of the two kinds of blasto- 
spheres are just as marked from my point of view as from 
those of the two above-mentioned naturalists. 
The production of the so-called invagination-gastrula from 
the bilamellar placula, which we have assumed as the primary 
stage, is not difficult to understand. It took place simply by ac- 
cumulation of fluid between the two cell-layers, by which these 
were gradually more and more separated from each other and 
finally inflated into a spherical form, so that one half of the 
wall of the sphere was formed by the ectoderm and the other 
by the entoderm. The right to conceive of the formation of 
the invagination-gastrula in this fashion may be based upon 
the fact that all possible degrees of transition between the 
simple bilamellar plate and its more or less considerable sphe- 
rical inflation by the development of a segmentation-cavity, 
occur in the ontogeny of different Metazoa. 
At any rate, it is for the present just as permissible and 
justifiable to endeavour to derive the invagination-blastula 
from the bilamellar plate in the manner here indicated, as, on 
the contrary, to adopt the opposite course, which has hitherto 
been usually followed, and to regard the plate as a coenogenetic 
product of metamorphosis of an original blastula. In accord- 
ance with our view, however, we must again, in this place, 
put the question, Could the transformation of the so-called 
placula into the blastula-form confer upon the developing 
embryo certain advantages which are of a nature to give pro- 
bability to the occurrence of such a ccenogenetic process ? 
To this I have but little to answer, and hence it appears to 
me that here lies a weak point of the hypothesis. How- 
ever, it may be noted that the production of the spherical 
form may have given rise to an increased mobility of 
the incipient stages of development, of course under the 
