6 Dr. W. Saleiisky on IliichiVa Gastraa Theory. 



we may suppose such a stage to occur in Stroufjijhtx filaria 

 and CuruUnnns. In all Ncmatoila an embryo consisting of 

 two layers is t'ormcd alter the conclusion ol" the jirocess of seg- 

 mentation. From the outer layer is formed the body-wall, 

 from the inner one the intestine. The production of the in- 

 testinal cavity occurs at the time of the formation of the body- 

 cavity. 



The researches of Kt)walevsky o\\ the embryology of Sa<jitta* 

 establish beyond doubt that in the embryology of this worm 

 we may admit a (!r</.s7/-«/(/-stage. 



The statements of lliickel that a GaMnda-stage occurs in 

 the Bryozoa do not agree with the known investigations. 

 From the researches of Nitschef, Clapar^dej:, and ^lecznikoff 

 we know that at any rate in the Bicellarice [Bugida) no 

 stomachal cavity is formed in the larva. As to the embryonic 

 development of the C//j)/ionai/fei<-V\k(i larva?, which, as is Avell- 

 known, possess an intestinal canal, we have no information. 

 In the postembryonic developmental history of the Bryozoa, 

 which has been better investigated than their embryonic de- 

 velopment, we find no state which has any resemblance to the 

 Gastnila-stage. It is well known that here the intestinal 

 canal (polypide) is developed in a very different manner from 

 other animals. 



In the class Gepli}Tea, if PJioronis is to be referred to it, a 

 Gasfrula-stage occurs in that worm. 



Hiickel's statement that Euaxes in its ontogeny passes 

 through a Gastrula-stagc, is decidedly not correct. The re- 

 markable investigations of Kowalevsky, upon which Hiickel 

 depends, best prove this. From the embryology of the Oli- 

 gocha^ta we learn that cases may occur in which, in the same 

 group of animals, one animal ])asses through a decided Gas- 

 tnda-stage in its ontogeny, while another does not. This 

 fact alone sufiiciently proves that, in the demonstration of the 

 Gastrcea theory, we can by no means be contented with a few 

 representatives of the animal types. The two Oligochgeta 

 which Kowalevsky has selected as the subject of his researches, 

 Euaxes and Lumbricus (the ontogeny of Tuhifex is like that 

 of Euaxes) show veiy essential differences in their first em- 

 bryonic stages. In Euaxes the segmentation takes place in 

 the way which is so characteristic of some Vermes and Mol- 

 lusea. After the first cell-division four large spheres of seg- 

 mentation are formed, upon which a great quantity of smaller 



♦ '•' Erabrvolopiscbe Studien an Wiirnifrii uud Arthropoden," in M^m. 

 de I'Acad. Imp. de St. Potersb. tome xvi. 

 t Zeitsclii-. fiir wiss. Zool. Bd. xx. 

 \ Ihid. Hd. xxi. 



