18 Proceedings of the Royal Physical Society. 



typical as any that we could find in a Coelenterate or a Nematode. Such 

 observations are of interest from two points of view. 



(1) They accentuate the probability that it is wrong to think of the 

 motor apparatus of the Vertebrate as built up of independent sets of units — 

 nervous (neurons) and muscular. If the conception of units is made use of 

 at all as a working hypothesis, the unit should be the complex consisting of 

 central nerve cell, nerve fibre, and muscle cell, which we may appropriately 

 term a myoneuron, such as is diagrammatical ly shown in Fig. 5. The 



Fig. 5. 



Primitive Myoneuron of a Vertebrate, 

 c/. Contractile fibrils ; 

 m. Myoblast lying in myotome ; 

 n.c. Motor nerve cell in spinal cord ; 

 n.f. Motor nerve fibre. 



motor end-plate of the fully-formed muscle fibre is of course the remnant of 

 the protoplasmic body of the muscle cell — the part of that cell which has not 

 become replaced by contractile elements. 



(2) They fit in with the reference back of the nervous and muscular 

 systems of the Vertebrate to an epithelial condition in a far back ancestral 

 form. The data of Vertebrate embryology, when due weight is attached to 

 the developmental phenomena of the lower groups, seem to me to indicate 

 with as near certainty as we can hope for in such things, that the myotome 

 of the typical Vertebrate have been developed from enterocoelic pouches of 

 the type still persisting in the ontogeny of Amphioxus. The main muscles 

 of the Vertebrate have then upon this view become evolved out of the 

 epithelial walls of pouch -like outgrowths of the archenteric or ccelenteric 

 wall. 



