SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 191 



that in the median plane they almost surround the border of the com- 

 missure (Plate 6, Fig. 41). Comparing the condition in adult Amia 

 (Fig. 40) with this, we see that there the cells lie about the border of 

 the recess of the mesencephalon, which extends forward above the pos- 

 terior commissure, but they are always morphologically posterior and 

 dorsal to the commissure. In Figure 40 one cell (a) is shown wliich 

 might be considered as ventral to the commissure. This meseucephalic 

 recess is not much deVeloped in Polypterus. 



The explanation of this position of the cells in Polypterus is probably 

 embryological. In the development of the commissure the nerve-fibres, 

 growing across from the opposite sides, have passed above and among 

 the already established cells, instead of anterior to them, as in other 

 ganoids and indeed in most vertebrates. As a result of this condition 

 many of the cells are crowded out of the median plane and lie laterally in 

 the tectum about the anterior border of the ventricle (Plate 6, Fig. 42, 

 cl. tcf. rfx.). 



The cells are multipolar, having two principal processes in addition 

 to the axon. The axon of the cell is frequently found projecting into 

 the ventricle ( Fig. 42, axJ"). Usually, however, it takes a less direct 

 course through the brain tissue toward the median plane, uniting with 

 other axons befoi-e emerging. The more prominent of the other two 

 processes (Fig. 42, ax.") has been traced into the middle layer of the 

 tectum. The third process has been followed only a short distance. 

 The cells are much larger than in Amia, having an average length of 

 4.5 micra and breadth of 30 micra. 



4. AciPENSER. I have had no opportunity to study Acipenser, having 

 no material at my disposal, but through the courtesy of Dr. J. B. John- 

 ston I have been permitted to examine his sections. In general the 

 conditions are much as in Amia and Polypterus, except that the 

 cells, as might be expected in a fish of such great size, are much ' 

 larger. 



Goronowitsch ('88) was the first to observe the optic reflex cells in 

 ganoids. I quote from his description of the brain of Acipenser 

 ruthenus ( p. 55-3). " In den proximalsten Abschnitten des Tectum wird 

 die Kbrnerschict diinner. Medial von derselben erscheinen sehr grosse, 

 ziemlich dicht zerstreute Ganglienzellen. Die feinea Fortsiitze dieser 

 Zellen verlaufen radial zur Oberflache des Tectum, es schien mir dabei, 

 dass sie mit Opticus Fasern in Verbindung treten, welches Verhalten 

 aber ohne zweckmassige, speciell darauf gerichtete histologische Unter- 

 suchung nicht festgestellt werden kann." He believed these cells to 



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