ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 341 



I pointed out that beneath the medullary groove the 

 epiblast and hypoblast were not separated by any interposed 

 mesoblast. Along the line (where the mesoblast is defi- 

 cient) which forms the long axis of the embryo, a rod-like 

 thickening of the hypoblast appears (PL XIV, figs, 7 a and 

 7 b, ch and ch'), first at the head end of the embryo, and 

 gradually extending backwards. This is the rudiment of the 

 notochord ; it remains attached for some time to the hypo- 

 blast, and becomes separated from it first at the head end of 

 the embryo, and the separation is then carried backwards. 

 This thickening of the hypoblast projects up and comes in 

 contact with the epiblast, and in the later stages with bad 

 (especially chromic-acid) specimens the line of separation 

 between the epiblast and the thickening may become a little 

 obscured, and might possibly lead to the supposition that a 

 structure similar to that which has been called the " axis 

 cord " was present. In all my best (osmic-acid) specimens the 

 line of junction is quite clear ; and any one who is aware 

 how easily two separate masses of cells may be made indistin- 

 guishably to fuse together from simple pressure will not be 

 surprised to find the occasional obscurity of the line of junc- 

 tion between the epiblast and hypoblast. In the earlier 

 stage of the thickening there is never in the osmic-acid pre- 

 parations any appearance of fusion except in very badly 

 prepared ones. Its mode of formation will be quite clear 

 without further description from an inspection of PL XIV, 

 figs. 7 a and 7 b, ch and ch' . Both are taken from one 

 embryo. In fig, lb, the most anterior of- the two, the 

 notochord has become quite separated from the hypoblast. 

 In fig. 7 a, ch, there is only a very marked thickening of 

 hypoblast, which reaches up to the epiblast, but the thick- 

 ening is still attached to the hypoblast. Had I had space 

 to insert a drawing of a third section of the same embryo 

 there would only have been a slight thickening of the hypo- 

 blast. In the earlier stage it will be seen, by referring to 

 figs. 6 a and 6 b, that there is no sign of a thickening of the 

 hypoblast. My numerous sections (all made from embryos 

 hardened in osmic acid) showing these points are so clear that 

 I do not think there can be any doubt whatever of the noto- 

 chord being formed as a thickening of the hypoblast. Two 

 interpretations of this seem possible. 



I mentioned that the mesoblast appeared to be primitively 

 formed as two independent sheets, split off, so to speak, from 

 the hypoblast, one on each side of the middle line of the 

 embryo. If we looked upon the notochord as a third median 

 sheet of mesoblast, split off from the hypoblast somewhat later 



