PE TROM YZONTIA 



55 



hand, the development of the anterior myotomes, the absence of a 

 horizontal septum subdividing the myotomes, the cranial nerves, 

 the skull which does not enclose the vagus, and other points of 

 structure, not to mention such histological characters as the absence 



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of medullated nerve-fibres, etc., and the ciliated groove and thyroid 

 gland of the Ammocoete larva, point clearly to the conclusion that 

 the Cyclostome stem originated from the common Craniate trunk 

 at a point far below that at which the known classes of the 

 Gnathostomata began to diverge. Attempts have indeed been made 



