EVOLUTION OF ORGANS IN THE CHORDATA. 269 



Balfour and his school) is seen in the centre dividing the 

 section into an anterior and posterior half. This cavity is 

 continuous below with the pericardium. The artery of the 

 arch is on the posterior side of the cavity, or as it is better to 

 call it, from the destination of its walls, of the muscle tube of 

 the arch. The branchial processes grow out first on the pos • 

 terior side, and along their base appears a vein which opens 

 dorsally into the artery. Similarly on the anterior side appear 

 branchial processes with an anterior vein, also opening into 

 the artery. The two veins become connected by two hori- 

 zontal commissures. In the adult the posterior vein becomes 

 disconnected from the anterior and unites with the anterior vein 

 of the arch behind it. 



The cartilaginous arch arises as a condensation of mesoderm 

 cells posterior to the muscle-plate. Between the upper and 

 lower venous commissures, where the muscle-tube is already 

 diminished in thickness, condensation of mesoderm cells takes 

 place also on the anterior side, and the two condensed masses 

 uniting, eliminate the muscle-tube between them. This separa- 

 tion of the muscle-tube does not take place dorsally and ven- 

 trally, because the cartilaginous arch bends inwards in those 

 regions. A central part of the muscle-tube is thus separated 

 and lies on the inner side of the arch ; it becomes the adductor 

 arcus visceralis. Both Gegenbauer and Vetter believe the 

 adductor mandibulse to be homodynamous with the adductor 

 arcus, but this is an error, the former is homodynamous with 

 the whole musculature of one (or more) arch. The external 

 middle portions of the tubes form the musculi interbranchiales ; 

 the dorsal, externally the constrictor superficialis, internally the 

 interarcuales. Other muscles come from the ventral portions. 

 The coracohyoid is a true body muscle, and has nothing to 

 do with visceral arches. 



The cartilage already described, the middle portion first 

 developed, forms the two middle internodes of the adult arch. 

 Above these dorsally is the basale, below the copulare. The 

 cartilage separates the adductor from the interarcualis above, 

 from the coracobrauchialis below. The cartilage is internal to 



