FORMATION OF THE LAYERS. 249 



my preliminary paper 1 , an illustration which is repeated on PL 

 7, fig. 2. 



The number of cells on the floor of the cavity differs con- 

 siderably in different cases, but these cases come under the 

 category of individual variations, and are not to be looked upon 

 as indications of different states of development. 



In many cases especially large cells are to be seen on the 

 floor of the cavity (PI. 7, fig. 2, b d\ In my preliminary paper 2 

 the view was expressed that these are probably cells formed 

 around the nuclei of the yolk. This view I am inclined to 

 abandon, and to substitute for it the suggestion made by Dr 

 Schultz, that they are remnants of the larger segmentation cells 

 which were to be seen in the previous stages. 



Plate 7, figs. 2, 3, 4 (all sections of this stage) shew the 

 different appearances presented by the floor of the segmentation 

 cavity. In only one of these sections are there any large number 

 of cells upon the floor ; and in no case have cells been observed 

 imbedded in the yolk forming this floor, as described by Dr 

 Schultz 3 , but in all cases the cells simply rested upon it. 



Passing from the segmentation cavity to the blastoderm 

 itself, the first feature to be noticed is the more decided differ- 

 entiation of the epiblast. This now forms a distinct layer 

 composed of a single row of columnar cells. These are slightly 

 more columnar in the region of the embryonic swelling than 

 elsewhere, and become less elongated at the edge of the blasto- 

 derm. In my specimens this layer was never more than one 

 cell deep, but Dr Schultz 4 states that, in the Elasmobranch 

 embryos investigated by him, the epiblast was composed of 

 more than a single row of cells. 



Each epiblast cell is filled with yolk-spherules and contains 

 a nucleus. Very frequently the nuclei in the layer are arranged 

 in a regular row (vide PI. 7, fig. 3). In the later blastoderms of 

 this stage there is a tendency in the cells to assume a wedge-like 

 form with their thin ends pointing alternately in opposite 



1 Loc. cit. 



- Qy. Journal of Micros. Science, Oct. 1874. [This Edition, No. V.] 



3 Loc. cit. Probably Dr Schultz, here as in other cases,"has mistaken nuclei for 

 cells. 



4 Loc. cit. 



B. 17 



