DEVELOPMENT OF THE KI.ASMOBKANCH FISIfKS. 8 1 



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 indistinguishably 

 to fuse together from simple pressure will not be surprised to 

 find the occasional obscurity of the line of junction between the 

 epiblast and hypoblast. In the earlier stage of the thickening 

 there is never in the osmic-acid preparations any appearance of 

 fusion except in very badly prepared ones. Its mode of for- 

 mation will be quite clear without further description from 

 an inspection of PI. 3, figs, ja and "jb, ch and c/i . Both are 

 taken from one embryo. In fig. jb, the most anterior of the 

 two, the notochord has become quite separated from the hypo- 

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

 hypoblast, which reaches up to the epiblast, but the thickening 

 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 hypoblast. In the 

 earlier stage it will be seen, by referring to figs. 6a and 6b, that 

 there is no sign of a thickening of the hypoblast. My numerous 

 sections (all made from embryos hardened in osmic acid) shew- 

 ing these points are so clear that I do not think there can 

 be any doubt whatever of the notochord 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 llic 

 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 than the 

 other two, we should avoid having to admit its hypoblastic origin. 



Professor Huxley, to whom I have shewn my specimens, 

 strongly advocates this view. 



H. 6 



