Johnston, MorpJwlogy of the Head. 241 



visual and cutaneous apparatus in primitive vertebrates. The 

 specialization of the secondar}' and tertiary centers, visual and 

 cutaneous, we see beginning in the fishes and going on in the 

 higher classes of vertebrates. 



We may sum up these several lines of evidence in the fol- 

 lowing hypothesis. The retitta belongs morpJiologically as ivell as 

 physiologically to the cutaneous sensory system. It cojisists of the 

 general ciitaneous ganglion and corresponding centers belojiging to 

 nenrouiere ii, zuhich have passed out of the brain ivall in order to 

 remain exposed to the light. The optic tract is Jiomologoiis ivith 

 the internal arcuate fibers of the cutaneous centers. 



Reference may be made here of a possible comparison of a 

 part of the retina with the acustico-lateral system. It will be 

 seen that in the above there is supposed to be included in the 

 neural tube when it rolls up all that area which at the level of 

 the acustico-lateral anlage is supposed to be left out. The 

 thought quickly presents itself that perhaps the rod and cone 

 cells are homologous with the neuromast cells in the acustico- 

 lateral sense organs, the supposition being that both are derived 

 from cells of the original neural plate which have the power to 

 develop into special sense cells. The presence of sense cells in 

 the saccus vasculosus of lower vertebrates [f)'], 9, 8) gives color 

 to this supposition although they are situated at the median line 

 instead of the lateral border of the neural plate. The writer 

 sees one morphological objection to this modification of the 

 above hypothesis; namely, that the rod and cone cells are bipolar 

 ganglion cells and not merely sense cells. Another objection 

 is that we have no evidence on the physiological side of close 

 relation between the neuromast cells which are specialized for 

 pressure sense and the rods and cones which are sensitive to 

 light waves. Since we have experimental evidence of a close 

 physiological relation between the general cutaneous ganglion 

 cells and the rods and cones, the writer is inclined to accept the 

 hypothesis in the more simple form stated obove. 



The writer has read with great interest Boveri's effort (10) 

 to connect the eye of vertebrates with the simple invertebrate 

 eye through the pigment spot of Amphioxus. The hypothesis 



