DEVELOPMENT OF ELASMOBRANCH FISHES. 49 



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 IV. fig. 8 c). 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. IV. fig. 8 b, n. al. The cells constituting this mass even- 

 tually become converted into the lateral and ventral walls of the 

 alimentary 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 IV. 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 \ In my 

 preliminary account ^ it was stated that this was a condi- 

 tion of the mesoblast at a very early period, and that it was 

 probably its condition from the beginning. Sections are now in 

 my possession which satisfy me that, from the very first, the 

 mesoblast arises as two distinct lateral masses, one on each side 

 of the axial line. 



1 Professor Lieberkiilin {Gesellschaft zu Marburg, Jan. 1876) finds in Mam- 

 malia 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. 



^ Loc. cit. 



B. 4 



