92 BULLETIN OF THE 



border of the retinal infolding iinmediately tinder neath 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 inversion 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 tvith 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 2^lO'Ce where the op)tic 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 ivhich are di- 

 rected toivards the nuclei. It is equally evident that there is a gradual 

 shiftin" in the reorion 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 original distribution, whereby the nerve-fibres are excluded from 

 their primitive mode of connection witli the sensory cells. If this is tlie 

 true explanation of the cause of the shifting of the nerve-fibres, it off'ers 

 a valid argument in favor of the secondary (i. e. recent) origin of the pre- 

 nuclear bacilli. 



But if these bacilli are not the original rods, what has become of the 

 latter 1 Were it not for this marked influence of the developing bacilli 

 on the course of tlie optic-nerve fibres, one might have assumed that tlie 

 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- 



* Til is explanation of the peculiar position of the optic nerve as it emerges from 

 the eye was first suggested to me by Dr. Whitman. 



