98 BULLETIN OF THE 



able from previously presented arguments — these post-nuclear eyes were 

 developed from fanctional monostichous eyes, the deep ends of whose reti- 

 nal cells were directly connected to the nerve-fibres, the fibres should 

 retain t'heir connection with the deep ends of the cells, and should ex- 

 hibit, even in advanced stages, a course similar to that pursued by the 

 nerve in " pre-nuclear " eyes at an early stage (Fig. 1 ). Instead of that 

 they traverse the post-retinal layer, which may have acquired the func- 

 tions of an optic ganglion in addition to its duties as a pigment-layer. 

 The narrowness of the tapetal band makes it probable that most of the 

 nerve-fibres pass around its margins in making their way from the retina 

 to the post-retinal layer. Although this is a modification of, it is not 

 fundamentally different from, the condition in pre-nuclear eyes. In the 

 latter the fibres are collected into a single bundle at the deep end of the 

 pocket, and therefore emerge at the posterior harder of the eye only ; in 

 the post-nuclear type the fibres pass over the lateral margins of the pocket 

 (and the outer edges of the tapetum) as well as its deep end (compare 

 Figs. 28, 29) before they are joined into a single trunk. The only real 

 difference between the two is in the share which the " post-retinal " layer 

 appears to take in the formation of eyes of the "post-nuclear" type. It 

 is conceivable that this condition may have been brought about gradually 

 during the stages of inversion, — that the nerve-fibres of the aborted half 

 of the eye, instead of undergoing complete atrophy, acquired relations 

 with the persistently functional parts of the retina and their nerve-fibres, 

 and thus influenced the course of the latter. 



Cambridge, June 22, 1886. 



