So DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



in Osseous fishes is the true medullary groove ; although Oel- 

 lacher and Kuppfer appear to have conclusively proved that it 

 does not become converted into the medullary canal. The 

 chief difference between the Dog-fish and Osseous fish, in ad- 

 dition to the point of difference about the medullary groove, is 

 that the epiblast is in the Dog-fish a single layer, and not 

 divided into nervous and epidermic layers as in Osseous fish, 

 and this difference is the more important, since, throughout the 

 whole period of development till after the commencement of 

 the formation of the neural canal, the epiblast remains in Dog- 

 fish as a one-cell-deep layer of cells, and thus the possibility 

 is excluded of any concealed division into a neural and epi- 

 dermic layer, as has been supposed to be the case by Strieker 

 and others in Birds. 



Development of the Embryo, 



After the embryo has become definitely established, for 

 some time it grows rapidly in length, without externally under- 

 going other important changes, with the exception of the ap- 

 pearance of two swellings, one on each side of its tail. 



These swellings, which I will call the Caudal lobes (figs. 8 

 and 9, t s), are also found in Osseous fishes, and have been 

 called by Oellacher the Embryonal saum. They are caused by 

 a thickening of mesoblast on each side of the hind end of the 

 embryo, at the edge of the embryonic rim, and form a very 

 conspicuous feature throughout the early stages of the develop- 

 ment of the Dog-fish, and are still more marked in the Torpedo 

 (PI. 3, fig. 9). Although from the surface the other changes 

 which are visible are very insignificant, sections shew that the 

 notochord is commencing to be formed. 



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 deficient) which forms 

 the long axis of the embryo, a rod-like thickening of the hypo- 

 blast appears (PI. 3, figs, ^a and jb, ch and ch'), first at the 

 head end of the embryo, and gradually extends backwards. This 

 is the rudiment of the notochord ; it remains attached for some 

 time to the hypoblast, and becomes separated from it first at 



