DEVELOPMENT OF THE ELASMOBRANCH FISHES. 79 



disappeared, though I do not think it would be very difficult to 

 do so. Nor will I attempt to discuss the question whether the 

 condition with a large amount of food-yolk (as seems more 

 probable) was twice acquired once by the Elasmobranchii and 

 Osseous fishes, and once by Reptiles and Birds or whether only 

 once, the Reptiles and Birds being lineal descendants of the 

 Dog-fish. 



In reference to the former point, however, I may mention 

 that the Batrachians and Lampreys are to a certain extent 

 intermediate in condition between the Amphioxns and the Dog- 

 fishes, since in them the yolk becomes divided during segmen- 

 tation into lower layer cells and epiblast, but a modified invo- 

 lution is still retained, while the Dog-fish may be looked upon 

 as intermediate between Birds and Batrachians, the continuity 

 at the hind end between the epiblast and hypoblast being 

 retained by them, though not the involution. 



It may be convenient here to call attention to some of the 

 similarities and some of the differences which I have not yet 

 spoken of between the development of Osseous fish and the 

 Dog-fish in the early stages. The points of similarity are (i) 

 The swollen edge of the blastoderm. (2) The embryo-swelling. 

 (3) The embryo-keel. (4) The spreading of the blastoderm 

 over the yolk-sac from a point corresponding with the position 

 of the embryo, and not with the centre of the germ. The growth 

 is almost nothing at that point, and most rapid at the opposite 

 pole of the blastoderm, being less and less rapid along points 

 of the circumference in proportion 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 con- 

 verted into the medullary canal in the same way as in Birds 

 and all other vertebrates, except Osseous fishes, where it comes 

 to nothing, and is, in fact, a rudimentary structure. 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 appears 



