96 BULLETIN OF THE 



On the other hand, if it be assumed that there has been an inversion, 

 some of the steps in the process appear more easily explainable. Figures 

 25-29 have been drawn to indicate a possible line of development by 

 inversion, having two stages (Figs. 25, 26) common to this and the "pre- 

 nuclear " type. The direct cause of the beginning of the inversion has 

 been assumed in this instance to be a gradual shifting in the position 

 of the original lens, rather than the appearance of a second lens bring- 

 ing light from a different direction. The shifting — so one may reason 

 — is accompanied by a gradual atrophy of one side of the retina, the 

 simultaneous development of a tapetum, and a peculiar modiiication in 

 the course of the fibres of the optic nerve which arise from the persistent 

 portion of the retina, 



A lens changing in its relation to the retina, as indicated in the 

 figures, might easily allow a part of the eye to remain functional during 

 the process of inversion ; but alone it would aff'ord no explanation of the 

 cause of the inversion, since it would not begin to have an influence 

 (similar to that ascribed to the new lens in " pre-nuclear " eyes) until 

 the change in the direction of the axis of the retinal depression (the 

 thing to be explained) had become sufficient to make some of the retinal 

 cells parallel to the axis of the lens. It must be admitted, then, that, 

 alone, this shifting of the lens is not an adequate explanation. It may 

 be, however, that the formation of a tapetum is the cause, in con- 

 nection with the shifting of the lens, both for the atrophy of one side 

 of the retina, and the inversion of the other side. 



If the formation of a reflecting structure (tapetum) were accompanied, 

 by a slight shifting on the part of the lens, the tapetum would practi- 

 cally cut off the light from one face of the retina and reflect it to the 

 sensitive elements of the opposite face. That would result in an atrophy 

 of the part robbed of light, and an increased development of that on 

 which additional (reflected) light fell. 



The direction of the reflected rays may, in addition, have influenced 

 the shape of the retina : if the tapetum were at first a straight band 

 parallel with the original axis of the optic depression (compare Fig. 26), 

 the light falling upon it would be reflected at nearly equal but very 

 oblique angles, no matter upon \^at portion of the band it fell. If, how- 

 ever, the deep portion of the band became slightly curved (concave towards 

 the persistent portion of the retina), — as would be altogether natural 

 with an increase in the thickness of the retinal layer on one (functional) 

 side, and a corresponding decrease in thickness on the other (atrophied) 

 side, — the rays reflected from the curved portion of the tapetum would 



