234 Journal of Comparative Neurology and Psychology. 



branchial placodes of doubtful character described by v.Kupffer. . 

 Do all of these organs belong to the one segment represented 

 by the anterior head cavities, or are most of them to be regarded 

 as organs without segmental significance? It seems to the 

 writer that the nervous structures represent more segments than 

 have been preserved in the mesoderm. In other words, there 

 are preserved vestiges of nerve structures belonging to segments 

 whose entodermal and mesodermal organs have disappeared for 

 the most part. The physiological relationships and the struc- 

 ture of these organs should give the clue to their segmental re- 

 lationships. The division of the brain wall of this region into 

 four neuromeres in embryos of selachians, teleosts and the chick 

 gives a clue to the number of segments. 



A full consideration of the visual organs is important for 

 any scheme of segmentation. The paired lateral eyes arise from 

 the lateral surface of the brain tube and remain attached by a 

 hollow stalk to the second neuromere of Locy — in higher verte- 

 brates the cerebral segment. The stalk becomes replaced by 

 the optic tract whose primary place of ending in the brain is the 

 optic lobes. Comparative studies (37) show that there are two 

 epiphyses in vertebrates, one caudal to the other, both of which 

 possess the structure of visual organs with nerve cells and fibers 

 often present in the adult. These two organs are descfibed as 

 hollow outgrowths of the dorsal wall of the diencephalon. The 

 anterior one in Petromyzon, Acipenser, and reptiles sends nerve 

 fibers into the ganglion habenulae ; the posterior one in Petro- 

 myzon, selachians, teleosts, Amia, Anura and reptiles sends its 

 fibers back to the posterior commissure. It is probable that 

 they end in the optic lobes. Such being the general descrip- 

 tion of the visual organs, have they any relation to other sensory 

 structures and what is their morphological and segmental sig- 

 nificance? 



The lateral eyes are physiologically related to the skin and 

 the general cutaneous sensory system. Both retina and skin 

 receive stimuli from the external world and both are sensitive 

 to light. It has been shown experimentally by Parker (102) 

 that the skin of the frog is sensitive to light and that in a degree 



