284 SCOTT. ' [Vol. I. 



clined to think, is a secondary one, brought about in connec- 

 tion with the development of the sense-organs of the skin, which 

 do not occur in connection with the spinal nerves, and have 

 evidently extended from the head to the trunk. In the anur- 

 ous Amphibia, as described by Spencer (38), the process has 

 extended a step farther, and the nerve-trunk, as well as the 

 ganglion, is split off from the epiblast, and this can hardly be 

 regarded as the primary mode of formation. Certainly the en- 

 tire resemblance of the post-auditory cerebral nerve-roots to the 

 dorsal roots of the spinal nerves, and the connection of these 

 cranial roots by means of a continuous commissure with the 

 spinal nerve, is, to say the least, suggestive of a much closer 

 homology than Beard is inclined to allow them. This sugges- 

 tion is further strengthened in the c-a.'s,^. oi Petromyzon by the 

 apparent presence of ventral roots, represented by the hypo- 

 glossus. In the Selachians these ventral elements have been 

 suppressed, or, perhaps, never differentiated. Beard's view rests 

 upon the assumption that the ganglia of the cranial nerves 

 originated simultaneously with the so-called " branchial sense- 

 organs." But it is equally probable that these ganglia primarily 

 arose entirely from the nervous axis, just as do the spinal gan- 

 glia, and that the sense-organs originated later in connection 

 with the ganglia. That the ontogeny should have been abbre- 

 viated, and that the two sets of organs in most existing types 

 should arise together, is certainly nothing strange. It does not 

 at all follow from this that the cranial and spinal nerves are 

 identical in plan, as indeed they clearly are not, or that the 

 cranial nerves were originally such as the spinal nerves are now. 

 On the contrary, Balfour's view seems much more probable, 

 that the two classes of nerves are differentiations along diverging 

 lines from a primitive form common to both. It may very well 

 prove to be the case that the post-auditory cerebral nerves con- 

 form in the course of their development much more closely to 

 the type of the spinal nerves than do the anterior cranial 

 nerves, as would indeed be expected from the much greater dif- 

 ferentiation of the organs which the anterior nerves supply. 



III. The Sense-Organs. 



The development of the organs of sense in Petromyzon offers 

 several peculiarities, most of which are, however, simply due to 



