36 FORMATION OF THE LAYERS. 



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. ill. fig. 2, h d). In my preliminary 

 paper ^ 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 III. figs. 2, 3, 4 (all sections of this stage) show 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^, 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 columuar 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^ 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. Yery frequently the nuclei in the layer are ar- 

 ranged in a regular row (vide PI. III. 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 alter- 

 nately in opposite directions. This arrangement is, however, 

 by no means strictly adhered to, and the regularity of it is 

 exas^orerated in Plate iii. fio^. 4. 



The nuclei of the epiblast cells have the same characters as 

 those of the lower layer cells to be presently described, but 

 their intimate structure can only be successfully studied in 



^ Qy. Journal of Micros. Science, Oct. 1874. 



2 Loc. cit. Probably Dr Scbiiltz, here as in other cases, has mistaken nuclei 

 for cells. 

 ^ Loc. cit. 



