78 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



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 shew 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 alimentary 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 

 one of the steps by which a direct mode of formation comes 

 to be substituted for an indirect one by involution. We find, 

 in fact, in the Dog-fish, that the cells from which are derived 

 the mesoblast and hypoblast come to occupy their final position 

 in the primitive arrangement of the cells during segmentation, 

 and not by a subsequent and secondary involution. 



This change in the mode of formation of the alimentary 

 canal is clearly a result of change of mechanical conditions from 

 the presence of the large food-yolk. 



Excellent parallels to it will be found amongst the Molljasca. 

 In this class the presence or absence of food-yolk produces not 

 very dissimilar changes to those which are produced amongst 

 vertebrates from the same cause. 



The continuity of the hypoblast and epiblast at the em- 

 bryonic rim is a remnant which, having no meaning or function, 

 except in reference to the earlier mode of development, is 

 likely to become lost, and in Birds no trace of it is any longer 

 to be found. 



I will not in the present preliminary paper attempt hypo- 

 thetically to trace the steps by which the involution gradually 



