MUSEUM OF COMPARATIVE ZOOLOG\. 87 



able cause of its existence, in the followiag 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- 

 incr the present condition. One is. What could have been the advantage 

 in the assumption of the inverted position of the retinal cells in rela- 

 tion to the direction of the Avaves of the light-stimulus 1 The other, 

 How could the retina have remained functional during the whole of 

 the involution-process which accompanied the formation of the neural 

 tubel 



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 1 



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, end, 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- 



