OlV THE DEVELOPMENT OF THE ELASMOBRANCH FISUKS. 339 



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

 to be substituted for an mdirect 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 Mol- 

 lusca. 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 develop- 

 ment, 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 

 hypothetically to trace the steps by which the involution 

 gradually 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 are to a certain extent intermediate in 

 condition between the Amphioxus and the Dog-fishes, 

 since in them the yolk becomes divided during segmenta- 

 tion into lower layer cells and epiblast, but a modified involu- 

 tion 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 hypo- 

 blast 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 — (1) The swollen edge of the blastoderm. (2) The 

 embryo-swelling. (3) The embryo-keel. (4) The spreading 

 of the blastodern 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 



VOL. XIV. NEW SER. Z 



