476 Prof. S. Apdthy on 



The most cogent proof that the ancestral form of the Metazoa 

 was the Gastrwa — an animal with intestinal cavity and oral 

 aperture, composed of ectoderai 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 

 {BlasTJEa), 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 tliat 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 Blasta;a. 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 araceboid, /. 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 diflerentiation of the BlastuIa-eeWs, 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 Blastiva 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. 



Kuw 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 



