Dr. G. Biitschli on the Gastreea- Theory. 373 
The most usual view, which, as is well known, Hickel first 
endeavoured to establish, holds this primitive form to be the 
so-called gastrula, or, translated into phylogenetic language, the 
gastrea. ‘The conception of the phylogenetic origin of this 
gastrea is also conformable to this view ; it was produced, as 
indeed may so frequently be observed ontogenetically, by the 
invagination of a one-layered blastula or blastosphere. The 
deviations from this primitive course of development of the 
gastrula which occur in the ontogeny of numerous Metazoa 
may then be explained by the assumption of secondary 
deviations, changes of the original course of development. 
In opposition to this view different ideas were put forward, 
especially by Ray Lankester * and Metschnikoff +, who, not- 
withstanding certain differences, agreed in this, that they dis- 
puted the originality of the production of a bilamellar form by 
invagination, and thus endeavoured to deprive the so-called 
invagination-gastrula of its significance as a primitive form. 
In the place of this latter bilamellar form they sought to set 
one which also sometimes occurs in ontogeny, namely the 
so-called planula-form, which is destitute of a primitive 
mouth. Ray Lankester thought that we might regard as the 
most primitive that form of the planula which is furnished 
from the first with a central cavity; while Metschnikoff 
expressed himself in favour of the view that those planula- 
forms are the most primitive which are originally (¢. e. after 
bilamellarity has been preduced) solid, and only subsequently 
acquire an intestinal cavity by the separation of the central 
entodermal cell-mass. ‘he difference in the views of the 
two naturalists is chiefly caused by differences in their specu- 
lative opinions as to the processes of nutrition which led to 
the production of a special nutritive entodermal cell-layer. 
But upon this point we shall hereafter have to enter more 
into detail. 
Considering these contradictory views and the small pro- 
spect that is nowadays offered to us of attaining with our 
speculations to any thing really elucidatory, it might perhaps 
seem desirable to suppress a third view, to some extent inter- 
mediate between those above referred to._ If I nevertheless 
venture to develop it here briefly, 1 may plead that it pre- 
sented itself to me very unexpectedly in the course of other 
investigations, and that, after I had long pursued it, it 
* KE, Ray Lankester, “Notes on Embryology and Classification,” 
Quart. Journ. Mier. Sci. n. s. vol. xvii. 
+ E. Metschnikoff, “ Spongiologische Studien,” Zeitschr. fiir wiss, 
Zool. Bd. xxxii., and also “ Vergleich, embryol, Studien,”’ 2¢d, Bd. xxxyi, 
and XXXVil. 
