I04 Journal of Comi'auative Neukolo(;y. 



stated by Dohin, seems to the writer doubtful. The other 

 statement, that the entrance of the sensory roots of the 

 vagus and glossopharyngeus into the medulla results in a de- 

 cided segmentation of the latter, seems to have much evi- 

 dence behind it, and though the acceptance of this evidence 

 will perhaps make it necessary to regard the vagus as poly- 

 merous, yet the same reasons enforce the homology between 

 the vagus, facial and trigeminal and the true spinal nerves. 

 The difficulties in the way of a satisfactory solution of 

 the nerves of the eye-muscles increase with research. The 

 fact that they primarily have root ganglia and the pres- 

 ence of a chiasm in the fourth prevent the acceptance 

 of Rabl's theory of derivationfrom the trigeminus. The 

 organs of sense cannot be divorced from the nervous 

 system, and not the least important step in the solu- 

 tion of the problem of cephalic metamerism will be 

 the determination of the locus and nature of the organs of 

 special sense. An excellent general review of this field 

 is given by Professor C. A. Whitman. (') The comparison 

 of the segmental sensory organs of annelids with the organs 

 of special sense generally, is certainly suggestive. And 

 while it seems to the writer that there can be no doubt that 

 the pineal body has functioned as an eye in many cases, 

 it does not appear that its unpaired character need at all 

 militate against the theory that the vertebrate eye is an 

 excessively modified descendent of a non-specialized sensory 

 organ, like the segmental sensory organs of Vermes. That 

 the paired visual organs are older, phylogenetically speaking, 

 than those of either of the other special senses seems probable. 

 May it not prove that the paired eyes were developed before 

 the primitive nerve plate became a tube and that when the 

 invagination of the neural epithelium did occur the germina- 

 tive retinal cells were included in the involution, and that the 



1 Biological Lectures delivered at the \Larine Biological Lalioralury of Woods' Holl, 

 1890, p. 27. 



