MUSEUM OF COMPARATIVE ZOOLOGY. 87 
able cause of its existence, in the following portion of the paper, devoted 
to theoretical considerations. 
What have been the causes, and what is the real significance, of the 
hypodermal infolding accompanying the formation of ocelli ? 
The following speculations are an attempt at the solution of these prob- 
lems. It is not supposed that they offer a complete explanation of the 
phenomena, but it is hoped that they may stimulate criticism on the part 
of future observers, which will ultimately lead to a satisfactory elucidation 
of the conditions. 
The case of ocelli with pre-nuclear bacilli, in which there has been an 
involution with inversion of the retinal layer, will be considered first. 
One meets here a problem similar to that which is encountered in 
endeavoring to explain the origin of the retina in vertebrates. If the 
retina in the ancestors of vertebrates was a patch of ectoderm in its 
normal position, then there are two questions to be settled in explain- 
ing the present condition. One is, What could have been the advantage 
in the assumption of the cnverted position of the retinal cells in rela- 
tion to the direction of the waves of the light-Stimulus? The other, 
How could the retina have remained functional during the whole of 
the involution-process which accompanied the formation of the neural 
tube? : 
Here, in the “ pre-nuclear eyes,” the same questions arise: If the retina, 
which is formed by a process of inversion, was once a normally located 
portion of the ‘‘hypodermis,” how could it have remained functional 
during the process of inversion, and what could have been the motive 
which led to the inversion ? 
The question of the immediate cause may perhaps be more readily 
answered in the case of vertebrates than here ; for in vertebrates the ulti- 
mate inversion of the retinal cells is only a necessary consequence of a 
much more fundamental change, — the involution of the central nervous 
system, — which may find its adequate explanation in something (e. g. the 
protection of the nervous system) very remotely, if at all, connected with 
the functions of the eye. But in the case of spiders’ eyes it is different. 
The retina is formed comparatively late in embryonic life, and, so far 
as is yet known, independently of any such neural infolding. Unless, 
then, the retinal inversion can be connected with the formation of the 
cephalic portion of the central nervous system, the cause of this remark-- 
able complication must be sought in some advantage secured to the eye 
itself. It is not necessary that the motive be one that is constantly oper- 
