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 



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 



