Warren H. Lewis 265 
of the brain tissue or of the retina has apparently no influence then on 
the differentiation of the optic stalk. 
In these transplanted eyes the optic nerve appears to rise from the 
ganglionic layer and in some of the eyes passes through the choroidal 
fissure and into the optic stalk (see Figs. 17, 19, 20, 21, 22, and 23), 
to be traced in some specimens along the entire length of the optic stalk 
into an attached piece of transplanted brain, or the nerve may pass 
through the fissure and thence into the medulla of the host.” The nerve 
may penetrate the retina in various places other than the choroidal 
fissure. In one of the eyes it extended as far as the outer molecular 
layer, in which layer it continued for some distance as a distinct bundle. 
In a few of the eyes the nerve ends abruptly at the pigment layer and 
seems to have been unable to penetrate either into or through it (Fig. 
30). In many of the transplanted eyes the nerve passes through the 
retina at one place or another into the pigment layer, it then continues 
in this layer as a distinct bundle of fibers between the pigment cells” 
(Fig. 29). It may continue in this manner for long distances, one-half 
- to three-fourths the circumference of the eye, and end abruptly here 
without leaving the eye. 
The nerve may penetrate the pigment layer and run into the mesen- 
chyme (Fig. 30) or may even enter the medulla.“ And even in the 
medulla the nerve can be traced as a distinct compact bundle through 
many sections (Fig. 31). The nerve may sometimes split as it passes 
through the retina into two or more separate bundles (Figs. 17 and 30). 
In a few instances the nerve instead of passing through the retina ex- 
tends directly from the ganglionic layer across the cavity of the eye and 
through the pupil into the mesenchyme.” Such variations in the optic 
nerve suggest at once that the axis cylinders are outgrowths from the 
cells of the ganglionic layer and that they follow the path of least re- 
sistance which, under normal conditions, would lead through the cleft 
and along the optic stalk into the brain. When the normal relations 
are disturbed, as in transplantation, the path of least resistance is dis- 
turbed with resulting variations in the course of the nerve. 
Where the optic nerve passes directly from the ganglionic layer across 
the cavity of the eye it would seem as though some disturbance in the 
arrangement of the flattened ends of Miiller’s fibers caused an imperfect 
* Lewis, Am. Jour. of Anat., Vol. VI, 1907, p. 471, Figs. 14 and 15. 
8 Tbid., Figs. 20 and 21. 
4 Tbid., Figs. 14 and 15. 
Ht... Wigs: 20 and) 2 
