xxxviii Journal of Comparative Neurology. 



arch, is divided into two epidermal rods which passing forward sur- 

 round the eye. They form later the supra- and infra-orbital mucus 

 canals. 



6. In connection with the general thickening of the anterior and 

 exterior surface of the hyoid arch, there is found at its inferior end a 

 thickening below the cleft on the mandibular arch, which in part gives 

 rise to the mandibular part of the lateral organs. 



7. Each branchial thickening later undergoes a further differen- 

 tiation. At the place where the ganglion of the corresponding nerve 

 attaches to it, along the superior and posterior edges of the visceral 

 cleft, a branchial organ is formed; six on each side of the body. 



8. Above each branchial organ there separates, in the form of a 

 rod, from the superior part of the branchial thickening at the level of 

 the notocord, the part destined for the lateral organs. Their forma- 

 tion is simultaneous with the development of the little rami of the 

 suprabranchial nerves. 



9. The lateral part which at first separates imperfectly gives rise 

 to the lateral line of the trunk. Three protons, scarcely distinct, of 

 the lateral organs corresponding to the second, third, etc. branches of 

 the vagus nerve constitute its proximal part. 



10. Thus the system of the lateral organs of Selachiens is primi- 

 tively composed of the following separate parts : of the protons of the 

 mucus canals, (a) supraorbital, (b) infraorbital, and (c) mandibular; of 

 the portions above the suprabranchial rami, (d) of the ramus buccalis 

 (ramus otkus), (e) of the glosso-pharyngeus, (/) of the first branch of 

 the vagus — the two latter forming the parietal canals, — (g) of the 

 second branchial ramus of the vagus ; (h) finally of the proton of the 

 lateral line properly so-called. 



After a review of the relations of the lateral organs in all of the 

 Ichthyopsida the author concludes that in all members of this group a 

 part of the general sensory epidermal thickening is split off to form 

 the auditory and branchial thickening which subsequently develops 

 into the organ of hearing, the branchial and lateral organs, very much 

 as described above for Selachians. This conclusion is very import- 

 ant for the definition of the metameric significance of the lateral 

 organs. For obviously these facts oppose the doctrine that the later- 

 al organs originate in a metameric manner. The metameric arrange- 

 ment is purely secondary, as a result of preexisting arrangements. 

 These organs therefore cannot be used as a criterion to define the 

 primitive vertebrate metamerism. C. J. H. 



