DEVELOPMENT OF ELASMOBRANCH FISHES. 211 



In stage K, four if not five are open to the exterior, and the 

 rudiments of six, the full number, have appeared \ Towards the 

 close of stage K there arise, from the walls of the 2nd, 3rd, 

 and 4th clefts, very small knob-like processes, the rudiments of 

 the external gills. These outgrowths are formed both by the 

 lining of the gill-cleft and by the adjoining mesoblast'^ 



From the mode of development of the gill-clefts, it appears 

 that their walls are lined externally by hypoblast, and therefore 

 that the external gills are processes of the walls of the alimen- 

 tary tract, i.e. are covered by an hypoblastic, and not an epiblastic 

 layer. It should be remembered, however, that after the gill- 

 slits become open, the point where the hypoblast joins the 

 epiblast ceases to be determinable, so that some doubt hangs 

 over the above statement. 



The identification of the layer to which the gills belong is not 

 without interest. If the external gills have an epiblastic origin, 

 they may be reasonably regarded^ as homologous with the ex- 

 ternal gills of Annelids ; but, if derived from the hypoblast, this 

 view becomes, to say the least, very much less probable. 



Segmentation of the Head. 



The nature of the vertebrate head and its relation to the 

 trunk forms some of the oldest questions of Philosophical 

 Morphology. 



The answers of the older anatomists to these questions 

 are of a contradictory character, but within the last few 

 years it has been more or less generally accepted that 

 the head is, in part at least, merely a modified portion 

 of the trunk, and composed, like that, of a series of homo- 

 dynamous segments*. While the researches of Huxley, Parker, 

 Gegenbaur, Gotte, and other anatomists, have demonstrated 

 in an approximately conclusive manner that the head is com- 

 posed of a series of segments, great divergence of opinion 

 still exists both as to the number of these segments, and 



1 The description of stage K and L, pp. 77 and 78, is a little inaccurate with 

 reference to the number of the visceral clefts, though the number visible in the 

 hardened embryos is correctly described. 



2 Vide on the development of the gills, Schenk, Sitz. d. k. Akad. Wien, 

 Vol. Lxxi., 1875. 



^ Vide Dohrn, Ursprung d. Wirhelthiere. 



* Semper, in his most recent work, maintains, if I understand him rightly, 

 that the head is in no sense a modified part of the trunk, but admits that it is 

 segmented in a similar fashion to the trunk. 



