92 BULLETIN OF THE 
border of the retinal infolding «mmediately underneath, the “ lentigen.” 
Upon the development of the bacilli the fibres emerge farther and farther 
back from the surface of the head, until finally a considerable interval 
separates the nerve from the lentigenous cells (Figs. 10, 23, 24, 20). 
This is exactly what might have been expected if the eye had been de- 
veloped phylogenetically by the tnversion of a layer of cells which were 
already in functional activity before the process of inversion began, and the 
deep ends of which were connected with the optic nerve.* It is also consist- 
ent with the formation at the deep ends of the retinal cells of SECONDARY 
bacilli, which may be regarded as the physical cause of a recession (onto- 
genetic) of the place where the optic nerve emerges. 
If the fibres of the optic nerve were originally joined to the proximal 
ends of the sensory cells, it is natural that they should have retained 
this connection for a longer or shorter period after the beginning of the 
involution which finally inverted the retina. The nerve-fibres are ulti- 
mately connected to post-bacillar parts of the retinal cells. There can 
be no doubt that the formation of the bacilli is a progressive process ; 
they are not begun throughout their whole extent at the same time, but, 
beginning at the originally deep ends of the retinal cells, they increase 
in length by successive additions to the ends of the rods which are di- 
rected towards the nuclei. It is equally evident that there is a gradual 
shifting in the region to which the nerve-fibres are distributed, so that 
this region is always post-bacillar. Nothing seems more reasonable, in 
view of these facts, than that the secondary condition of the nerve-fibre 
distribution results from the gradual development of bacilli in the region 
of the orzginal distribution, whereby the nerve-fibres are excluded from 
their primitive mode of connection with the sensory cells. If this is the 
true explanation of the cause of the shifting of the nerve-fibres, it offers 
a valid argument in favor of the secondary (1. e. recent) origin of the pre- 
nuclear bacilli. 
But if these bacilli are not the original rods, what has become of the 
latter? Were it not for this marked influence of the developing bacilli 
on the course of the optic-nerve fibres, one might have assumed that the 
new bacilli were not absolutely new structures, but only the original 
bacilli migrated from one end of the retinal cells to the other, pari passu 
with the process of retinal inversion, being therefore new only in the 
sense that they occupy new positions. Such a view seems, for the rea- 
* This explanation of the peculiar position of the optic nerve as it emerges from 
the eye was first suggested to me by Dr. Whitman. 
