Die Discogastrula unsers Knochenfisches in diesem Stadium der vollen Ausbildung stellt nunmehr eine kreisrunde Kappe dar, welche wie ein gefüttertes Mützchen fast die ganze obere Hemisphäre der hyalinen Dotterkugel eng anliegend bedeckt (Fig. 65). Der Ueberzug des Mützchens entspricht dem Exoderm (_e_), sein Futter dem Entoderm (_i_). Ersteres besteht aus drei Schichten von kleineren Zellen, letzteres aus einer einzigen Schicht von grösseren Zellen. Die Exodermzellen (Fig. 77) messen 0.006 - 0.009 Mm., und haben ein klares, sehr feinkörniges Protoplasma. Die Entodermzellen (Fig. 78) messen 0.02 - 0.03 Mm. und ihr Protoplasma ist mehr grobkörnig und trüber. Letztere bilden auch den grössten Theil des Randwulstes, den wir nunmehr als Urmundrand der Gastrula, als "_Properistoma_" oder auch als "RUSCONI'schen After" bezeichnen können. Der letztere umfasst die Dotterkugel, welche die ganze Urdarmhöhle ausfüllt und weit aus der dadurch verstopften Urmund-Oeffnung vorragt. Footnote 165: The references in this quotation are to the figures in the original. My objections to the view so lucidly explained in the passage 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 Elasmobranchii the hypoblast cells occupy, even before the close of segmentation, the position which, on Professor Haeckel's view, they ought only eventually to take up after being involuted from the whole periphery of the blastoderm. (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 segmentation 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 asymmetry of the blastoderm; an asymmetry which is unquestionably also present in the blastoderm of most Osseous Fishes, though not noticed by Professor Haeckel in the investigations recorded in his paper.