6Q FORMATION OF THE LAYERS. 



78) messen 0,02 — 0,03 Mm. und ihr Protoplasma ist mehr grobkornig 

 und triiber. Letztere bilden aiich den grossten Theil des Rand- 

 wulstes, den wir nunmehr als XJrmundrand der Gastrula, als 

 " Projoeristoma " oder auch als '' RuscoNi'schen After " bezeiclmen 

 konnen. Der letztere umfasst die Dotterkugel, welcbe die ganze 

 Urdarmhohle ausfiillt und weit aus der dadurch verstopften Urmimd- 

 Oeffnung vorragt. 



My objections to the view so lucidly explained in the pas- 

 sage just quoted, fall under two heads. 



(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 these 

 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 invagina- 

 tion 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 segmenta- 

 tion 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- 



