64 FORMATION OF THE LAYERS. 



which primitively corresponds in position with the blastopore 

 ur anus of Rusconi, causes the asymmetry of the gastrula invagi- 

 nation, since it is not possible for the part of ovum which will 

 become the ventral wall of the alimentary canal, and which is 

 loaded with food-yolk, to be invaginated in the same fashion as 

 the dorsal wall. From the asymmetry, so caused, follow a large 

 number of features in vertebrate development, which have been 

 worked out in some detail in my paper already quoted \ 



Prof. Haeckel, in a paper recently published ^ appears to 

 imply that because I do not find absolute invagination in 

 Elasmobranchs, I therefore look upon Elasmobranchs as mili- 

 tating against his Gastr^a theory. I cannot help thinking 

 that Prof. Haeckel must have somewhat misunderstood my 

 meaning. The importance of the Gastrsea theory has always 

 appeared to me to consist not in the fact that an actual in- 

 gi'owth of certain cells occurs — an ingrowth which might have 

 many different meanings^ — but in the fact that the types of 

 early development of all animals can be easily derived from 

 that of the typical gastrula. I am perfectly in accordance with 

 Professor Haeckel in regarding the type of Elasmobranch 

 development to be a simple derivative from that of the gastrula, 

 although believing it to be without any true ingrowth or inva- 

 gination of cells. 



Professor Haeckel* in the paper just referred to published 

 his view upon the mutual relationships of the various vertebrate 

 blastoderms. In this paper, which appeared but shortly after 

 my own^ on the same subject, he has put forward views which 

 differ from mine in several important details. Some of these 

 bear upon the nature of food-yolk ; and it appears to me that 

 Professor Haeckel's scheme of development is incompatible 

 with the view that the food-yolk in meroblastic eggs is the 

 homologue of part of the hypoblast of the holoblastic eggs. 



The following is Professor Haeckel's own statement of the 



^ Quart. Joum. of Micr. Science, July, 1875. 



'•^ Die Gastrula u. Eifurchurig d. Thiere, Jenaische Zeitschrift, Vol. ix. 



2 For instance, in Crustaceans it does not in some cases appear certain 

 whether an invagination is the typical gastrula invagination, or only an invagi- 

 nation by which, at a period subsequent to the gastrula invagination, the hind 

 gut is frequently formed. 



4 Loc. cit. 



s Loc. cit. 



