646 



J. PLAYFAIK MCMUKiaCH. 



The position of the second arch articulating with the anterior 

 extremity of the trabecules appears at first an insuperable 

 objection to this theory ; but when we consider that it is 

 possible that concomitantly with the bending up of the trabe- 

 culae and the overgrowth of the cerebrum there was a carrying 

 forwards of this arch without any bending up, away out of the 

 difficulty is presented. The fact that nerves pass out through 

 openings in the walls of the skull formed by the trabecule does 

 not appear to be of any weight against the theory, since it is 

 well known that when in the process of extension cartilage 

 meets with a nerve or blood-vessel instead of contracting and 

 closing or interfering with the action of that structure, a wide 

 foramen is left through which it may pass freely. In favour 

 of the theory this may be said, that it reduces to a natural 

 group structures which, viewed in any other light, are ex- 

 ceedingly abnormal, and is what one might expect from what 

 occurs in the persisting ancestral forms.' 



III. The Notochord and Vertebral Column. 



In Stage a (fig. 1) the notochord in the cranium is en- 

 sheathed by the parachordal cartilages. Below the medulla 

 oblongata it bends abruptly downwards at an angle of 45°. 



1 By this expression I mean Amphioxus and Ascidians, which, though pro- 

 bably not in the direct genealogical line, may still be recognised as typea of 

 tiie diicct anccstoi's. 



