228 Recent Literature. [March, 



We have in this work, for the first time in connected form, the 

 comparative embryology of the Ganoids, the researches of Salensky 

 on the sturgeon, and of A. Agassiz on the gar-pike, supplemented 

 by those of the author, assisted by Professor W. K. Parker and his 

 son, W. N. Parker, giving us a good idea of the development of two 

 principal types. In the sturgeon the segmentation of the yolk is 

 complete, but the embryo does not become folded off from the 

 yolk in the manner usual in Vertebrates, while the relation of the 

 yolk to the embryo is unlike that in any other known vertebrate. 

 Before hatching the embryo has, to a small extent, become folded 

 off from the yolk both anteriorly and posteriorly, and has also 

 become, to some extent, vertically compressed. Owing to these 

 changes, it resembles somewhat the embryo of a bony fish. Ac- 

 cording to Parker, in older larvae a very rudimentary gill appears 

 to be developed on the front walls of the spiracular cleft, while 

 the gill-papillae of the true branchial arches are of considerable 

 length. There is a suctorial disk, with slender papillae, which prob- 

 ably ultimately become the barbels, and a corresponding but tem- 

 porary one arises in the gar-pike. 



In the gar-pike, besides the discoveries, as respects the later stages, 

 made by A. Agassiz, the segmentation is total; but the early stages 

 of the embryo show a remarkable resemblance to those of bony 

 fishes. Both the head and tail become early folded off from the 

 yolk, as in bony fishes. The yolk in the gar forms a special 

 external yolk sack, instead of an internal dilatation of part of the 

 alimentary tract as in the sturgeon, and besides, in the gar it is 

 placed behind instead of in front of the liver, as in the sturgeon. 

 A knowledge of the mode of development of the Ganoids is, of 

 course, most important, since from them the Amphibia are sup- 

 posed to have been derived. But, as Balfour observes, there are 

 no very prominent Amphibian characters in the development of 

 either type, otherwise than a general similarity in the segmentation 

 and formation of the germ-layers. So that no light is thrown by 

 embryology on the origin of the Amphibia. In considering the 

 development of the Amphibia a good deal of stress is laid on the 

 resemblance between the mouths of the tadpole and the lamprey, 

 and Balfour thinks that these are not merely the results of more or 

 less similar habits. Says Balfour : " In dealing with the Ganoids and 

 other types arguments have been adduced to show that there was 

 a primitive vertebrate stock provided with a perioral suctorial 

 disc; and of this stock the Cyclostomata are the degraded, but at 

 the same time the nearest living, representatives. The resem- 

 . blanccs between the tadpole and the lamprey are probably due to 

 both of them being descended from this stock. The Ganoids, as 

 we have seen, also show traces of a similar descent ; and the re- 

 semblance between the larva of Dactylethra, the Old Red Sand- 

 stone Ganoids and Chimaera probably indicate that an extension 

 of our knowledge will bring to light further affinities between the 



