58 THE PINEAL ORGAN 



he discussed differences in the structure of the median and lateral eyes of 

 vertebrates and assumed that these different types of eyes of vertebrates 

 must have arisen as modifications of two types of skin — the vestigial 

 " pineal eye " being formed originally from a type of skin similar to that 

 of the invertebrates, consisting of a single layer of palisade or hypodermal 

 cells beneath the cuticle, which was supported internally by a layer of 

 connective tissue ; whereas the lateral, paired eyes arose from the neural 

 ectoderm and the stratified layers of squamous epithelium which form the 

 epidermis of vertebrates, combined with the connective tissue components 

 forming the outer tunics of the eyeball. He suggested also that the 

 " pineal eye " " developed first as an optic pit from the skin of the 

 ancestors of the vertebrates before that skin had assumed the vertebrate 



mes. 



Fig. 



ect. : 

 i.e. : 

 mes. . 

 p.e. : 



43. — Section of an Eye of Arca barbata. (After Rawitz.) 



ectoderm. r. : inner segment of cell containing 



interstitial cell, 

 mesoderm, 

 pigment cell. 



rod-like elements. 

 v.c. : visual cell of retina. 



type, i.e. before the palisade layer had become protected externally by 

 the mucous and horny layers." He also assumed that : 



1 . The retina is but a specialized portion of the epithelial layer of the 

 skin, between the cells of which the pigment granules from the subjacent 

 chromatophoral layer stream outwards under the influence of light. 



2. The retina and the chromatophoral layer must have been in 

 intimate and inseparable association through all the stages of evolution of 

 the eye. 



He believed that simple eyes first arose in pigmented areas on exposed 

 surfaces of the body in invertebrates, and that the action of light not only 

 produced and caused movement of pigment granules, but that the irrita- 

 tion of the hypodermal cells resulted in the formation of slime. This 



