CHAPTER VI. 

 Development of the Trunk during stages G to K. 



By the stage when the external gills have become conspicuous 

 objects, the rudiments of the greater number of the important 

 organs of the body are definitely established. 



Owing to this fact the first appearance of the external gills 

 forms a very convenient break in the Elasmobranch develop- 

 ment; and in the present chapter the history is carried on 

 to the period of this occurrence. 



While the last chapter dealt for the most part with the 

 formation of the main organic systems from the three embry- 

 onic layers, the present one has for its subject the gradual 

 differentiation of these systems into individual organs. In treat- 

 ing of the development of the separate organs a divergence 

 from the plan of the last chapter becomes necessary, and the 

 following arrangement has been substituted for it. First of 

 all an account is given of the development of the external 

 epiblast, which is followed by a description of the organs 

 derived from the mesoblast and of the notochord. 



External Epiblast 



During stages G to I the epiblast' is formed of a single 

 layer of flattened cells ; and in this, as in the earlier stages, it 

 deserves to be especially noticed that the epiblast is never more 

 than one cell deep, and is therefore incapable of presenting any 

 differentiation into nervous and epidermic layers. (PI. x. ficr. 

 1-5). 



1 Unless the contrary is stated, the facts recorded in this chapter apply 

 only to the genera Scyllium and Pristiurus. 



