No. 2.] 



HISTOGENESIS OF THE RETINA. 



417 



or less highly specialized ; the middle cell is often bipolar and 

 reaches from the first to the third. ^ 



1 The more rudimentary arrangement is when the sensory cell reaches from the 

 ectoderm to the brain, then in turn the brain cell sends a prolongation to the 

 muscle, thus making a very simple reflex circle. The beautiful researches of 

 V. Lenhossek and of Retzius on Tumbricus show this state of things. In the olfac- 

 tory organ we have a single cell reaching from the ectoderm to the brain as 

 shown by His and other investigators. The arrangement becomes more complex 

 in sense organs like the ear, taste-buds and eye. 



In the eye the chain is composed of three cells ; the rods and cones, the 

 bipolar cells of the inner nuclear layer and the ganglion cells of the retina. 

 Moreover we have in all probability nerves reaching from the brain to the retina 

 as indicated by Monakow and Ramon y Cajal. In this manner we can pass 

 through the whole system of sense organs and always find that the direction of 

 the activity of the cell is always indicated by the pole which originally communi- 

 cated with the exterior of the body. 



Fig. I. — Diagrammatic transverse section of tlie head of an embryo, to show the growing point 

 in the nervous system and the direction of the growth of the fiber. M, medullary canal ; E, eye ; O, 

 ear; N, nose ; C, cephelopod eye ; L, sense cells of the skin of Lumbricus. 



Fig. 2. — Longitudinal section of the cerebral hemispheres of Necturus. V, ventricle ; W, white 

 matter; G, grey matter extending from the ventricle to form a rudimentary certex. The growing 

 point and the direction of the nerve fiber is indicated. 



Difficulties are met with in the formation of the gray matter, and also the ganglia 

 of the encephalon, but I think a careful investigation of them has shown me that 

 the same also holds true in these parts. In the gradual change of gray matter 

 from the ventricle of the brain in lower animals to the cortex of the higher, the 

 cell undergoes a half revolution and the side which originally pointed toward the 

 ventricle now points toward the surface of the brain. The only serious obstacle 

 with which I have met are the cells of the dorsal ganglia of higher animals. The 

 cells continue to divide after they are separated from the neural crest, and more- 

 over they become unipolar. This must be viewed as a secondary change, as 

 originally they were bipolar. The general indication is that the pole which 

 gives rise to the nerve fiber is the original basal end of the cell. Figs, i and 2 

 indicate in a general way the points I have emphasized. The sense cells of the 

 skin of Lumbricus, as well as the cephalopod eye, are indicated in the diagram. 

 (For literature regarding these points, see Altmann, His, Merk, Rabl and others.) 



