FORMATION OF THE LAYERS. 283 



remains quite unintelligible to me how an ingrowth of cells from 

 a circumferential line, to form a layer which had no previous 

 existence, can be equivalent to, or derived from, the invagination 

 of a layer, which exists before the process of invagination begins, 

 and which remains continuous throughout 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 Gastraea 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 Gastraea theory in its general bearings. 



Observations upon the formation of the layers in Elasmo- 

 branchs have hitherto been very few in number. Those published 

 in my preliminary account of these fishes are, I believe, the 

 earliest 1 . 



Since then there has been published a short notice on the 

 subject by Dr Alex. Schultz 2 . 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. 



19 2 



