476 Prof. 8. Apathy on 
The most cogent proof that the ancestral form of the Metazoa 
was the Gastr@a—an animal with intestinal cavity and oral 
aperture, composed of ectoderm and endoderm, without meso- 
derm—is therefore wanting. WE REQUIRE THIS FORM, 
HOWEVER, AS A TRANSITIONAL STAGE, ONLY IF WE WOULD 
HAVE THE NEXT STEP IN THE SEQUENCE OF THE PHYLOGE- 
NETIC DEVELOPMENT, STARTING FROM THE BLASTULA-FORM 
(Lrasrz4), TO CONSIST IN AN INVAGINATION, 
It is true that for a Gastrula to arise by invagination is 
MECHANICALLY the simplest mode of further development, 
and therefore it is that ontogeny, which always strives after 
abbreviation and simplification, so often adopts this method, 
especially among the higher types; therefore, on the other 
hand, it is also natural and easily explainable that the next 
stage in ontogeny after the Blastula is the Gastrula without 
mesoderm. But the question arises whether a similar forma- 
tion of a Gastrula is also the PHYSIOLOGICALLY simplest 
possibility in the further development from the Blastwa. It 
does not appear to me that it is. The method of forming 
the endoderm which is physiologically the simplest, and 
therefore probably genealogically the oldest, is that which 
commences with immigration into the inner cavity of ectoderm 
cells, which have been forced out of connexion with the epi- 
thelium and have become ameceboid, @. e. formation of the 
endoderm by apolar multilocular inward growth (Metschni- 
koff). The growth is apolar, because only by subsequent 
adaptation (accumulation of yolk) could the original polarity 
of the ovum be so far increased as already actually to effect 
a differentiation of the Slastula-cells, a greater difference 
between the hypoblast and epiblast. The cells which pene- 
trated into the cavity of the blastula afterwards gradually 
arranged themselves again in the manner of an epithelium to 
form the endoderm, after the communication of the blastula- 
cavity with the exterior by means of the blastopore had fur- 
nished an incentive thereto. Perhaps an open lastea of 
this kind is even more archaic than the closed vesicle, and in 
that case the incentive alluded to would not have subse- 
quently occurred, but would have been present from the 
beginning. I would remind the reader of the development of 
Volvox, where the young but already perfect colonies close 
their opening only after leaving the parent. 
Now it cannot have happened either that all the immigrant 
cells were utilized for the formation of the endoderm, or that 
with the completion of the endoderm the immigration from 
the ectoderm at once came to an end; it is much more 
probable that the cells of the mesenchyma also should be 
