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 embryonic 

 layers, the present one has for its subject the gradual differentia- 

 tion of these systems into individual organs. In treating of the 

 development of the separate organs a divergence from the plan 

 of the last chapter becomes necessary, and the following arrange- 

 ment 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 fr-om the mesoblast and of 

 the notochord. 



External Epiblast. 



During stages G to I the epiblast 1 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 differentia- 

 tion into nervous and epidermic layers. (PI. 11, figs, i 5.) 



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

 the genera Scyllium and Pristiurus. 



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