DEVELOPMENT OF THE ELASMOBRANCH FISHES. 75 



tive, i.e. whether, when in the stage before this the mesoblast 

 is first formed, it is only on each side of the middle line that 

 the differentiation of the lower layer cells into hypoblast and 

 mesoblast takes place, I do not certainly know, but it is un- 

 doubtedly a very early condition of the mesoblast. The con- 

 dition of the mesoblast as two plates, one on each side of the 

 neural canal, is precisely similar to its embryonic condition in 

 many of the Vermes, e.g. Euaxcs and Lmnbricus. In these there 

 are two plates of mesoblast, one on each side of the nervous 

 cord, which are known as the Germinal streaks (Keimstreifen) 

 (vide Kowalevsky " Wurmern u. Arthropoden"; Mem. de I'Acad. 

 Imp. St Petersbourg, 1871). 



From longitudinal sections I have found that the segmen- 

 tation cavity has ceased by this stage to have any distinct 

 existence, but that the whole space between the epiblast and 

 the yolk is filled up with a mass of elongated cells, which 

 probably are solely concerned in the formation of the vas- 

 cular system. The thickened posterior edge of the blastoderm 

 is still visible. 



At the embryonic end of the blastoderm, as I pointed out 

 in an earlier stage, the epiblast and the lower layer cells are 

 perfectly continuous. 



Where they join the epiblast, the lower layer cells become 

 distinctly divided, and this division commenced even in the 

 earlier stage, into two layers ; a lower one, more directly 

 continuous with the epiblast, consisting of cells somewhat 

 resembling the epiblast-cells, and an upper one of more flat- 

 tened cells (PI. 3, fig. 4, m}. The first of these forms the 

 hypoblast, and the latter the mesoblast. They are indicated by 

 hy and m in the figures. The hypoblast, as I said before, re- 

 mains continuous with the whole of the rest of lower layer cells 

 of the blastoderm (vide fig. 7 b). This division into hypoblast 

 and mesoblast commences at the earlier stage, but becomes 

 much more marked during this one. 



In describing the formation of the hypoblast and meso- 

 blast in this way I have assumed that they are formed out 

 of the large mass of lower layer cells which underlie the epi- 

 blast at the embryonic end of the blastoderm. But there 

 is another and, in some ways, rather a tempting view, viz. 



