COMMISSURES OF THE BRANCHIAL NEUROMERES. 



79 



It is only in the second neuromere that it is anatomically a branch of the haemal 

 nerve; in the more posterior neuromeres it arises from the side of the neuromere 

 and entirely separate from the haemal nerve roots. 



Commissures. — The transverse commissures of the cord may be divided 

 into two sets: a. the primary, or haemal commissures, passing underneath the 

 epithelium of the embryonic median groove, and representing the primitive nerve 

 tracts uniting the right and left cords; and b. the secondary, or neural commissures, 

 crossing the neural fissure above the floor of the median groove. The neural 

 commissures, phyllogenetically and ontogenetically form much later than the 

 haemal, and only after the median groove becomes deep enough to bring the 

 superior median margin of the two cords into contact. 



The anterior and posterior haemal commissures are separated by an opening 

 in the floor of the neural canal through which the neuroglia passes to the underlying 

 lemmatochord (Figs. 64 and 68.) 



The anterior haemal commissure consists of several indistinct bundles. So 



an.n.co. m.n.co. p.n.co. 



m.n.co. 



p.n.co. 



a.h.co. 



Fig. 64. — Sagittal section of two anterior branchial neuromeres, showing the relation of the principal neurones 



and fiber tracts to the cross commissures. 



far as could be determined, the anterior portion consists of fibers from neurones 

 I; the middle portion from neurones A; and the posterior portion from neurones 

 B (Fig. 64). 



The posterior haemal commissure contains fibers from group J (anterior 

 bundle) and from group D (posterior bundle). 



The haemal commissures therefore contain, among fibers of undetermined 

 character, crossed motor axones and various collaterals. 



The neural commissures are three in number; an anterior, a middle and 

 a posterior one. (Fig. 64.) The sources of the fibers in the anterior neural 

 commissure could not be certainly determined. Those of the middle commissure 

 m.n.co., are derived from neurones, F, and those of the posterior commissure from 

 neurones E. No other fibers could be located in these commissures. The neural 

 commissures appear to be largely composed of association fibers. 



The Neuropile Centers. — The neuropile centers are dense masses of inter- 

 woven terminal dendrites. They appear in von Rath's preparations as dense 

 black masses of fine fibrils, and in methylene blue, as masses of fine blue dots or 

 lines, according to the character of the stain. 



