1317, 
formed from 1) the pigment layer of the retina, 2) from the brain 
tissue of another embryo, 3) from the ectoderm dorsal to the mid-brain 
region, 4) and from the cord of ectodermal cells forming the anlage 
of the nasal organ. 
Since in some of my experiments the retina was reversed either 
directly by turning it around or as a consequence of its development 
in the brain, we have in these cases an eye of an invertebrate type 
in that the rods and cones are directed peripherally instead of cen- 
trally. One may therefore believe that one factor which causes the 
development of the lens from the pigment layer at the normally inner 
pole of the eye is of an atavistic nature. The reversed position of 
the retinal cells may call forth potential properties of their protoplasm 
which at a phylogenetically earlier stage were dominant. This inter- 
pretation is offered at present only as a possibility. 
It is evident that by these experiments strong support is given 
to the theory of mutation, first expressed by NussBAum in 1894 and 
1896 (Verhandl. d. Anat. Gesellsch., 8. Versamml., p. 181, and 10. 
Versamml., p. 66) and later Arch. f. mikrosk. Anat., Bd. 52, p. 463 sq., 
MERKEL und Bonnet, Ergebnisse, 1902, p. 247, und Sitzungsber. der 
Niederrhein. Gesellsch. vom 16. Februar 1903. 
This subject will be considered more fully in a later paper. 
The Optic Nerve. 
Embryo 45, 3,5 mm. The left anterior end of the head region 
was removed. In this embryo the regenerated side of the brain is 
very thin and there is no trace of an eye on the left side. The optic 
stalk is greatly dilated at its cranial attachment, and its ventral part 
consists of a very thin layer of cells. The fibres of the optic nerve 
not being able to enter the ventral part of the optic stalk (at its di- 
lated cranial attachment) go into the same side of the brain. They 
pass as a bundle along the outer surface of the grey substance for a 
short distance and then obliquely forwards into the grey substance. 
The bundle is lost in the grey substance near the ventricle about the 
junction of its lower and middle thirds. 
This experiment shows that the fibres of the optic nerve may 
penetrate parts of the brain which they normally do not enter if they 
be diverted from their normal path. There may therefore be chemo- 
tactic forces which normally guide the nerve fibres along a special 
path toward a special center, but the energy of growth would seem 
to be in the neurone itself. When the normal paths of growth are 
impermeable, the nerve fibres expend their innate energy in invading 
