DEVELOPMENT OF THE ELASMOBRANCH FISHES. 77 



The second class are those with meroblastic ova and no anus 

 of Rusconi, and with an alimentary canal formed by the infold- 

 ing of the sheet of hypoblast, the digestive canal remaining in 

 communication with the food-yolk for the greater part of em- 

 bryonic life by an umbilical canal. 



This class includes the " Elasmobranchii," " Osseous fish," 

 " Reptiles," and " Aves." 



The mode of formation of the alimentary canal in the first 

 class is clearly the more primitive ; and it is equally clear that 

 its mode of formation in the second class is an adaptation due 

 to the presence of the large quantity of food-yolk. 



In the Dog-fish I believe that we can see, to a certain extent, 

 how the change from the one to the other of these modes of de- 

 velopment of the alimentary canal took place. 



In all the members of the first class, viz. " Amphioxus" the 

 "Lamprey," the "Sturgeon," and the "Batrachians," the epiblast 

 becomes continuous with the hypoblast at the so-called " anus 

 of Rusconi," and the alimentary canal, potentially in all and 

 actually in the Sturgeon (vide Kowalevsky, Owsjannikow, and 

 Wagner, Bulletin der Acad. d. St Petersbourg, Vol. XIV. 1870, 

 " Entwicklung der Store "), communicates freely at its ex- 

 treme hind end with the neural canal. The same is the case 

 in the Dog-fish. In these, when the folding in to form the 

 alimentary canal on the one hand, and the neural on the 

 other, takes place, the two foldings unite at the corner, where 

 the epiblast and hypoblast are in continuity, and place the two 

 tubes, the neural and alimentary, in free communication with 

 each other 1 . 



There is, however, nothing corresponding with the " anus of 

 Rusconi," which merely indicates the position of the involution 

 of the digestive canal, and subsequently completely closes up, 

 though it nearly coincides in position with the true anus in the 

 Batrachians, &c. 



This remarkable point of similarity between the Dog-fish's 

 development and the normal mode of development in the first 

 class (the holoblastic) of vertebrates, renders it quite clear 

 that the continuity of the epiblast and hypoblast in the Dog- 



1 This has been already made out by Kowalevsky, " Wurmern u. Arthropoden, " 

 loc. tit. 



