FORMATION OF THE LAYERS. 277 



which have been worked out in some detail in my paper already 

 quoted 1 . 



Prof. Haeckel, in a paper recently published 2 , appears to 

 imply that because I do not find absolute invagination in 

 Elasmobranchs, I therefore look upon Elasmobranchs as mili-- 

 tating against his Gastraea theory. I cannot help thinking that 

 Prof. Haeckel must have somewhat misunderstood my meaning. 

 The importance of the Gastraea theory has always appeared to 

 me to consist not in the fact that an actual ingrowth of certain 

 cells occurs an ingrowth which might have many different 

 meanings 3 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 invagination of cells. 



Professor Haeckel 4 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 5 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 

 scheme or type, which he regards as characteristic of mero- 

 blastic eggs, pp. 98 and 99. 



Jetzt folgt der hochst wichtige und interessante Vorgang, den ich als 

 Einstiilpung der Blastula auffasse und der zur Bildung der Gastrula 

 fiihrt (Fig. 63, 64) 6 . Es schlagt sich namlich der verdickte Saum der Keim- 

 scheibe, der " Randwulst " oder das Properistom, nach innen um und eine 

 diinne Zellenschicht wachst als directe Fortsetzung desselben, wie ein immer 



1 Quart. Journ. of Micr. Science, July, 1875. [This Edition, No. VI.] 



2 " Die Gastrula u. Eifurchung d. Thiere," Jenaische Zeitschrift, Vol. IX. 



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

 an invagination is the typical gastrula invagination, or only an invagination by which, 

 at a period subsequent to the gastrula invagination, the hind gut is frequently formed. 



4 Lac. cit. 5 Loc. cit. 



tf The references in this quotation are to the figures in the original. 



