BARGENT: THE OPTIC REFLEX APrAKATUS OF VERTEBRATES, 183 



relation to the optic ventricle have become considerably changed, and 

 consequently the course of the axons before entering the ventricle 

 has become modified and complicated. 



The cells, 70 to 100 in number, but showing in this respect, variation 

 from individual to individual, are for the most part aggregated in or 

 near the median plane, dorsal and posterior to the posterior commissure, 

 and are grouped about the anterior and dorsal walls of the mesencephalic 

 recess, which extends above the posterior commissure (Plate 5, Figs. 

 37, 38). From this region the cells extend caudad and laterad into the 

 optic tectum on either side of the median plane. Cephalad the cells 

 are closely aggregated (Figs. 34, 38), but caudad they are irregularly 

 distributed, a few stray cells sometimes occurring far out in the tectum 

 (Fig. 36). 



In the earlier stages, where all the cells are in immediate proximity 

 to the ventricle, the axons pass directly toward the ventricle and into it, 

 (Plate 4, Fig. 25). In the later stages the cells, in addition to having 

 undergone a seeming migration, have in many cases become partially 

 rotated by the development and consequent pressure of intervening cells. 

 Anteriad, where the cells are closely aggregated, those cells which lie 

 near the median plane have undergone a rotation of 90 degrees, so that 

 their axons now pass first laterad, and then caudad, describing a curve 

 and entering the ventricle near the median plane (Plate 4, Fig. 26). In 

 these cells, as seen in cross-sections of tliis region, tlie nucleus lies with 

 great uniformity at the mesal end of the cell, so that cells which lie 

 close together, but send their axons in opposite directions, have their 

 nuclei in juxtaposition (Plate 5, Figs. 3G, 37, 38). Tlie cells which lie 

 well caudad and laterad in tlie tectum send their fibres cephalad (Plate 4, 

 Fig. 26 ; Plate 5, Fig. 33), forming on either side of the median plane 

 loose fibre tracts which curve mesad towai'd the torus, the axons enter- 

 ing the ventricle near the median plane. The cells which still lie near 

 the ventricle send their axons directly into it (Plate 5, Figs. 33, 35). 

 In all these changes of the positions of the cells, the nucleus uniformly 

 retains its position at the end of the cell opposite to that from which 

 the axon emerges. 



The axons running proxiraally toward the ventricle pass between the 

 epithelial cells lining its surface and into the ventricle, and then straight 

 onward for a short distance ; then they are deflected toward the median 

 plane and coalesce with adjacent axons, eventually forming Reissner's 

 fibre (Plate 4, Fig. 26, /as. Beis.) by the union of all the axons of the 80 

 or 90 cells. 



