Johnston, Morphology of the Head. 239 



mere rostral to the optic vesicle has never been highly developed 

 and the tectum has early become specialized as a co-ordinating- 

 center. Whether this explanation of the centrifugal fibers shall 

 prove to be supported by further investigations of the cutaneous 

 centers, the main fact remains that the tracts which connect 

 the retina with the brain are closely similar to those which con- 

 nect the cutaneous centers with the tectum, and that the rise of 

 a large general cutaneous nerve (the profundus) in connection 

 with the tectum in the ontogeny indicates that the tectum was 

 originally a primary general cutaneous center. 



Following this line of thought it is possible to see in the 

 retina two chief layers which may correspond to the ganglia and 

 central nucleus respectively, and also to draw some comparisons 

 between the cells of the retina and those of the cutaneous sen- 

 sory centers. The outer nuclear layer with the rods and cones 

 may be thought to represent a general cutaneous ganglion. 

 The outer molecular layer is the place of meeting of the central 

 axones of these ganglionic cells with the dentrites of central cells, 

 and as such suggests comparison with the substantia gelatinosa. 

 The bipolar rod and cone cells of the inner molecular layer cor- 

 respond to the small cells characteristic of cutaneous centers 

 which in the cerebellum and acusticum of fishes become special- 

 ized as granules. The horizontal cells may perhaps be com- 

 pared with the simple II type cells in the cerebellum and other 

 cutaneous centers. The large cells of the ganglion cell layer 

 correspond to the large cells of cutaneous nuclei which give rise 

 to internal arcuate fibers. The comparison of the afferent and 

 efferent fibers has been made above. It may be said further 

 that there is even a correspondence between the retina and the 

 cutaneous centers in the relative position of the nerve elements. 

 Beginning at the original outer surface of the ectoderm we find 

 in each case the elements in the following order : the primary 

 receptive elements or ganglion cells, the substantia gelatinosa, 

 the small distributing cells of the sensory center, and the large 

 cells whose axones go to other brain centers. These fibers 

 travel at first next to the original inner surface of the ectoderm 

 and as they pass to the opposite side through the ventral com- 



