434 DEVELOPMENT OF ELASMOBRANCH FISHES. 



that the walls lining the clefts are entirely formed of hypoblast. 

 The clefts are formed successively 1 , the anterior appearing first, 

 and it is not till after the rudiments of three have appeared, that 

 any of them become open to the exterior. 



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

 rudiments of six, the full number, have appeared 2 . 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 3 . 



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 4 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 



1 Vide Plate 8. 



2 The description of stage K and L, pp. 292 and 293, is a little inaccurate with 

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

 hardened embryos is correctly described. 



3 Vide on the development of the gills, Schenk, Sitz. d. k. Akad. IVien, Vol. 

 LXXI. 1875. 



4 Vide Dohrn, Ur sprung d. Wirbelthiere. 



