MUSEUM OF COMPARATIVE ZOOLOGY. 95 
“pre-nuclear” 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 
the 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 
ease, and their non-development in the other, would then be left 
unexplained. 
There are grounds for supposing that the retention of the original bacille 
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. 
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 ? 
I have been unable to form any idea of how this condition could have 
been produced from a primitively monostichous retina with post-nuclear 
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 improbable 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.) 
