340 



F. M. BALFOUR, 



most rapid at the opposite pole of the blastoderm, being 

 less and less rapid along points of the circumference in pro- 

 portion to their proximity to the embryonic swelling. (5) The 

 medullary groove. 



In external appearance the early embryos of Dog-fish 

 and Teleostei are very similar ; some of my drawings could 

 almost be substituted for those given by Oellacher. This 

 similarity is especially marked at the first appearance of 

 the medullary groove. In the Dog-fish the medullary groove 

 becomes converted into the medullary canal in the same way 

 as with Birds and all other vertebrates, except Osseous fishes, 

 where it comes to nothing, and is, in fact, a rudimentary 

 organ. But in spite of Oellacher's assertions to the contrary, I 

 am convinced from the similarity of its position and appearance 

 to the true medullary groove in the Dog-fish, that the groove 

 which apj)ears in Osseous fishes is the true medullary groove ; 

 although Oellacher appears 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, 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 commence- 

 ment of the formation of the neural canal, the epiblast remains 

 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 

 undergoing other important changes, Avith the exception of 

 the appearance of two swellings, one on each side of its tail. 



These swellings, Avhich I will call the Caudal lobes 

 (figs. 8 and 9, / 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 development of the Dog-fish, and are still 

 more marked in the Torpedo (Pi. XIV, fig. 9). Although 

 from the surface the other changes which are visible are 

 very insignificant, sections show that the notochord is com- 

 mencing to be formed. 



