262 DEVELOPMENT OF ELASMOBRANCH FISHES. 



continuous conversion of lower layer cells into hypoblast along a 

 line leading towards the centre of the blastoderm. 



In the parts of the rim adjoining the embryo, the lower layer 

 cells, on becoming continuous with the epiblast cells, assume a 

 columnar form. At the sides of the rim this is not strictly the 

 case, and the lower layer cells retain their rounded form, though 

 quite continuous with the epiblast cells. One curious feature 

 of the layer of epiblast in these lateral parts of the rim is the 

 great thickness it acquires before being reflected and becoming 

 continuous with the hypoblast (PL 7, fig. 8c). In the vicinity 

 of the point of reflection there is often a rather large formation 

 of cells around the nuclei of the yolk. The cells formed here 

 no doubt pass into the blastoderm, and become converted into 

 columnar hypoblast cells. In some cases the formation of these 

 cells is very rapid, and they produce quite a projection on the 

 under side of the hypoblast. Such a case is represented in 

 PI. 7, fig. 8, 11. al. The cells constituting this mass eventually 

 become converted into the lateral and ventral walls of the alimen- 

 tary canal. 



The formation of the mesoblast has progressed rapidly. 

 While many of the lower layer cells become columnar and form 

 the hypoblast, others, between these and the epiblast, remain 

 spherical. The latter do not at once become separated as a 

 layer distinct from the hypoblast, and, at first, are only to be 

 distinguished from them through their different character, vide 

 Plate 7, figs. 6 and 7. They nevertheless constitute the com- 

 mencing mesoblast. 



Thus much of the mode of formation of the mesoblast can 

 be easily made out in longitudinal sections, but transverse sec- 

 tions throw still further light upon it. 



From these it may at once be seen that the mesoblast is 

 not formed in one continuous sheet, but as two lateral masses, 

 one on each side of the axial line of the embryo 1 . In my 



1 Professor Lieberktihn (Gesellschaft zu Marburg, Jan. 1876) finds in Mammalia a 

 bilateral arrangement of the mesoblast, which he compares with that described by me 

 in Elasmobranchs. In Mammalia, however, he finds the two masses of mesoblast 

 connected by a very thin layer of cells, and is apparently of opinion that a similar 

 thin layer exists in Elasmobranchs though overlooked by me. I can definitely state 

 that, whatever may be the condition of the mesoblast in Mammalia, in Elasmobranchs 

 at any rate no such layer exists. 



