338 p. M. BALFOUR. 



extent, how the change from the one to the other of these 

 modes of development 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, poten- 

 tially in all and actually in the Sturgeon {vide Kowa- 

 levsky, Owsjannikow, and Wagner, 'Bulletin der Acad. d. 

 St. Petersbourg,' vol. xiv, 1870, " Entwicklung der Store"), 

 communicates freely at its extreme hind end with the neural 

 canal. The same is the ease 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 hypo- 

 blast are in continuity, and place the two tubes, the neural 

 and alimentary, in free communication with each other.^ 



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- 

 fish is really the remnant of a more primitive condition, when 

 the alimentary canal was formed by an involution. Besides 

 the continuity between neural and alimentary canals, we have 

 other remnants of the primitive involution. Amongst these 

 the most marked is the formation of the embryonic rim, 

 which is nothing less than the commencement of an involu- 

 tion. Its form is due to the flattened, sheet-like condition 

 of the germ. In the mode in which the alimentary canal is 

 closed in front I shall show there are indications of the 

 primitive mode of formation of the alimentary canal ; and in 

 certain peculiarities of the anus, which I shall speak of later, 

 we have indications of the primitive anus of Rusconi ; and 

 finally, in the general growth of the epiblast (small cells of 

 the upper pole of the Batrachian egg) over the yolk (lower 

 pole of the Batrachian egg), we have an example of the 

 manner in which the primitive involution, to form the ali- 

 mentary canal, invariably disappears when the quantity of 

 yolk in an egg becomes very great. 



I believe that in the Dog-fish we have before our eyes 



' This has been already made out by Kowalevsky, " Wurmern u. Arthro- 

 podeu," loc. cit. 



