Slonaker, Eye of the Mole. 341 



some of them go between these cells ; but a greater number 

 run parallel and external to them (Fig. 34 and 35). These 

 cells in the optic tract resemble closely the ganglion cells of 

 the retina which have not developed axonic or dendritic pro- 

 cesses. They are, however, much shrunken and somewhat 

 irregular in contour. Their size compares favorably with that 

 of the cells of the inner nuclear layer. These cells of the 

 optic stalk can be traced into the eye where they are more or 

 less scattered and separated by nerve fibers and typical gang- 

 lion cells. Within the eye most of these cells lie with their 

 long axes parallel to the nerve fibers, but outside they are 

 arranged with the longest axis transverse to the course of the 

 nerve fibers. The original cells of the optic stalk thus form a 

 core around which the greater number of fibers are arranged. 

 A few fibers, however, run between the cells. 



A similar condition is found to obtain in other animals. 

 Studnicka (14) says, "In Petromyzon the lumen of the stalk 

 soon disappears, but its cells, or their descendants, persist 

 throughout life as an axial core of glia cells in the nerve." In 

 Protopterus he finds a similar condition, but the cells are fewer 

 and more scattered. In Necturus he found that the lumen of 

 the optic stalk persisted throughout life. He also says that in 

 other Amphibia there is either an axial mass of glia cells or 

 many similar columns scattered through the cross section of 

 the nerve, and that this is the condition generally found in the 

 higher vertebrates. 



In the European mole Kohl finds in the embryo 8. 5 mm. 

 long that the fibers of the optic nerve just after leaving the 

 eye lie in the groove of the optic stalk (choroid fissure) and 

 that some of them later enter the lumen of the stalk. The 

 greater number lie outside. At this age none of the fibers 

 extend to the brain. 



This agrees with Assheton's results (15) on the frog. He 

 says, "The optic nerve is developed independently of the 

 optic stalk, and at first entirely outside of it ; but on the break- 

 ing down of the stalk some of the nerve fibers grow in be- 

 tween the cells." 



