FORMATION OF THE LAYERS. 279 



(1) That the facts of development of the meroblastic eggs 

 of vertebrates, are not in accordance with the views here 

 advanced. 



(2) That even if these views be accepted as representing the 

 actual facts of development, the explanation offered of Ihese 

 facts would not be satisfactory. 



Professor Haeckel's views are absolutely incompatible with 

 the facts of Elasmobranch development, if my investigations are 

 correct. 



The grounds of the incompatibility may be summed up under 

 the following heads : 



(1) In Elasmobranchs the hypoblast cells occupy, even 

 before the close of segmentation, the position which, on Pro- 

 fessor Haeckel's view, they ought only eventually to take up 

 after being involuted from the whole periphery of the blasto- 

 derm. 



(2) There is no sign at any period of an invagination of the 

 periphery of the blastoderm, and the only structure (the embryonic 

 rim) which could be mistaken for such an invagination is confined 

 to a very limited arc. 



(3) The growth of cells to form the floor of the segmenta- 

 tion cavity, which ought to be part of this general invagination 

 from the periphery, is mainly due to a formation of cells from 

 the yolk. 



It is this ingrowth of cells for the floor of the segmentation 

 cavity which, I am inclined to think, Professor Haeckel has 

 mistaken for a general invagination in the Osseous Fish he has 

 investigated. 



(4) Professor Haeckel fails to give an account of the asym- 

 metry of the blastoderm ; an asymmetry which is unquestion- 

 ably also present in the blastoderm of most Osseous Fishes, 

 though not noticed by Professor Haeckel in the investigations 

 recorded in his paper. 



The facts of development of Osseous Fishes, upon which Pro- 

 fessor Haeckel rests his views, are too much disputed, for their 



