70 FORMATION OF THE LAYERS. 



existence, can be equivalent to, or derived from, the invagina- 

 tion of a layer, which exists before the process of invagina- 

 tion beg^ins, and which remains continuous throuo^hout it. 



If Professor Haeckel's views should eventually turn out to 

 be in accordance with the facts of vertebrate development, it 

 will, in my opinion, be very difficult to reduce them into 

 conformity with the Gastroea theory. 



Although some space has been devoted to an attempt to 

 refute the views of Professor Haeckel on this question, I wish 

 it to be clearly understood that my disagreement from his 

 opinions concerns matters of detail only, and that I quite 

 accept the Gastrgea theory in its general bearings. 



Observations upon the formation of the layers in Elas- 

 mobranchs have hitherto been very few in number. Those 

 published in my preliminary account of these fishes are, I 

 believe, the earliest \ 



Since then there has been published a short notice on the 

 subject by Dr Alex. Schultz^ His observations in the main 

 accord with my own. He apparently speaks of the nuclei of 

 the yolk as cells, and also of the epiblast being more than one 

 cell deep. In Torpedo alone, amongst the genera investigated 

 by me, is the layer of epiblast, at about the age of the last 

 described embryo, composed of more than a single row of cells. 



1 I omit all reference to a paper published in Russian by Prof. Kowalevsky. 

 Being unable to translate it, and the illustrations being too meagre to be in 

 themselves of much assistance, it has not been possible for me to make any 

 use of it. 



2 Centralblatt f. Med. Wiss. No. 33, 1875. 



