362 Herr B. Lwof( on the Formation of the 



fislies ; and Lacerta among the Keptiles. My investigations 

 upon the development of Amphioxus have ah-eady been 

 published"^. In the following pages it is my intention to 

 describe the results of my studies upon these developmental 

 processes in the above-mentioned Vertebrates, so far as is 

 necessary in order to establish my theory. Although I owe 

 this theory just as much to an attentive study of the literature 

 of the subject as to my own investigations, I shall never- 

 theless in the present paper leave the literature almost entirely 

 out of the question, in order not to overstep the limits of a 

 provisional communication. The discussion of the literature 

 dealing with the subject, as well as the full description of my 

 investigations, will follow in a detailed paper which is shortly 

 to appear. 



Before proceeding to the account of my results I would 

 draw attention to certain points which are of importance in 

 this question. In the first place let us consider whether the 

 school which is at present predominant is correct in regarding 

 the invagination in Vertebrates as gastrulation. In order to 

 answer this question it is necessary to ascertain v/hat is 

 actually to be understood by the term gastrulation, and to 

 discover what portion of the process is typical and what is 

 adventitious and unessential. 



Gastrulation is the term which was applied to the process 

 of invagination which leads to the formation of the enteric 

 cavity, whereby a distinct gastrula is constituted. But the 

 process of the formation of the intestine is not always accom- 

 plished by means of invagination ; it often happens that this 

 process takes place in such a way that the cells which subse- 

 quently form the intestine (endoderm cells) are overgrown 

 by the external (ectoderm) cells, and the enteric cavity after- 

 wards arises through divergence of the endoderm cells. This 

 process of enveloping growth is regarded by many investi- 

 gators i:s homologous with typical gastrulation, and, as I 

 think, justly, since in both cases the process essentially con- 

 sists in the fact that the endoderm cells which constitute the 

 intestine come to lie in the interior, and are surrounded by 

 the cells of the ectoderm. Whether this process is accom- 

 plished in the form of invagination or of enveloping growth 

 is of subordinate importance. It follows from this that the 

 only process which can be designated as gastrulation is that 

 by winch the endodermal elements are invaginated or over- 

 grown, by which consequently above all the formation of 

 the intestine is ushered in. We must hold fast to this cou- 



* Biol. Ceiitralblatt, Bd. xii, nos. 23 and 24. 



