ORIGIN OF JAW APPARATUS 403 
velopes and surrounded by the growing tissues of the body. The 
spreading walls of the medulla meet with resistance to their 
lateral travel, since they must displace other structures to make 
room for themselves. Crowded back from in front by the growing 
brain, they expand laterally against the resistance of the structures 
laterad of them. Since in the growing organism, as elsewhere, 
the forces of reaction are equal to the forces of action, the effect 
of the lateral pressures will inevitably tend to compress the wall 
of the medulla. We find evidence of this pressure in the relation 
of the nerve roots to the tip of the medulla. If in the transforma- 
tion of cord into medulla only the pressure of the brain in front 
had been operative, the nerve roots would be found leaving the 
edge of the arms of the Y on the same level and along a line curv- 
ing about the rounded anterior end of each arm. However, we 
find the arms have by lateral pressure been buckled upward and 
their inner and outer edges, originally anterior and posterior 
limits, bent ventrad, the inner edge curving laterad and the outer 
edge curving mesad. The root of the nasalis (palatine) nerve 
is thus forced below the inner edge of the arm and the ophthalmic 
occupies the apical position on the edge of the tip of the arm. On 
the outer edge of the arm we find the mandibular similarly curved 
downward and appearing to arise from the ventral and lateral 
surfaces of the arm. The maxillary trunk dominates the field on 
the dorsal surface of the arm. 
Up to the present only facts favorable to the view of the 
genetic connection of the jaw apparatus of Amphioxus with that 
of the Marsipobranch and through the latter with the true 
Gnathostomes have been brought forward. Are there any 
facts that do not harmonize? It would not bea problem awaiting 
solution if there were none, and furthermore, when one can work 
- out the solution of a problem in comparative anatomy and get 
all the facts and leave nothing unexplained or unaccounted for, 
the millennium will have arrived and comparative anatomy will 
probably lack incentive. 
Some investigators will raise the objection that the m. trans- 
versus abdominis of Amphioxus is not homologous in its anterior 
part with the circular muscles of the jaw and velar territory of 
Ammocoetes, for example. I think they are, as I shall explain 
