MUSEUM OF COMPARATIVE ZOOLOGY. 95 



" pre-iiuclear " type : the " post- nuclear " eye without tapetum (if such 

 exist) would, to a certain extent, represent a common antecedent of both 

 types, one of which might have been produced by the substitution of 

 new (pre-nuclear) for old (now become post-nuclear) bacilli, and the other 

 by the addition of a tapetum without change in the bacilli.* 



On theoretical grounds this seems to be the more probable phylo- 

 genetic course ; but upon this assumption — that there is an inversion of 

 ■,he retina — the explanation of the motive to the infolding offered above 

 for " pre-nuclear" eyes could not be simply extended to eyes of the post- 

 nuclear type, since the cause of the development of new bacilli in one 

 case, and their non-development m the other, would then be left 

 unexplained. 



There are grounds for supposing that the retention of the original bacilli 

 in "post-nuclear " eyes is due to the development of a tapetum, — a subject 

 to which I shall return directly. 



If the retina is not inverted, even a general comparison with the retina 

 of " pre-nuclear " eyes becomes difficult ; for the involution in that event 

 affects only the tapetal and post-retinal layers, not the retina itself. In 

 that case, too, the primitive condition of the eye must be assumed to 

 have been unlike any primitive conditions at present known ; viz., with 

 bacilli at the deep ends of the hypodermal (sensory) cells. f 



If there has been no inversion of the retina, the obstacles to an expla- 

 nation of the development are considerable. What can have been the 

 cause of an infolding which involves only the tapetal and post-retinal 

 layers, or of the peculiar outfolding between retinal and tapetal layers 1 

 I have been unable to form any idea of how this condition could have 

 been produced from a primitively monostichous retina with post-uucleixr 

 bacilli, consistently with the retention of the functional activity of the 

 eye during all the changes. Neither has it been possible to comprehend, 

 upon the same assumption, how the optic nerve came to emerge from the 

 post-retinal layer. 



* But if the retention of the original bacilli in the inverted retina was at first 

 directly dependent on the existence of a tapetum, this " common antecedent " con- 

 dition (without tapetum) would not have been realized, except as the result of a re- 

 gressive modification of the "post-nuclear" eyes, involving the disappearance of the 

 tapetum. 



+ It is not entirely impossible that eyes may have arisen which in the primitive, 

 uninverted condition possessed post-nuclear bacilli ; but it is very in)probable that 

 such was the case, because we have not at present, in any animal, a single instance 

 of monostichous eyes in which that condition obtains. (Compare the footnote to 

 pp. 88, 89.) 



