INFLUENCE OF LIGHT IN FORMATION OF PIGMENT 57 



of another embryo and developed by self-differentiation deeply beneath 

 the epidermis. Under these circumstances it gave rise to a lens which 

 developed from the edge of the optic cup, as in cases of regeneration of 

 an extirpated organ or part of an organ ' (Fig. 42). 



The latter experiment is of particular interest with reference to the 

 normal formation of a lens from a part of the optic vesicle as is the case in 

 the eyes of many invertebrates and in the " pineal eye " of some fishes 

 and reptiles ; although it must be born in mind that in the latter the lens 

 is developed from the distal or superficial wall of the optic vesicle, which 

 in the lateral eyes of vertebrates is invaginated to form the sensitive part 

 of the retina. In the experimental Triton embryo the regenerated lens 

 originated from the margin of the optic cup at the junction of the 

 invaginated layer with the ensheathing or pigmented layer of the retina, 

 and in the same situation as the vitreous or cellular, hypodermal lens of the 

 Dytiscus or Acilius larvae (Figs. 4 and 13). Incidentally it may be noted 

 that the pigment layer of the retina is in the same situation relative to the 

 " regenerated lens " as the zone of pigmented hypodermal cells beneath 

 the cuticle is to the vitreous lens of Dytiscus. 



The Influence of Light in the Formation of Pigment and the Production 

 of Physico-chemical Changes in the Superficial Layers of Epithelium. 



The whole question of lens formation and the specialization of refractile- 

 elements in relation with the receptive or sensory cells of the retina 

 appears to be linked with the action of light on the cuticle and hypodermal 

 cells of invertebrates and on the epidermis and the neuro-epithelial cells 

 of the retina in vertebrates. This action is associated with the formation 

 of pigment granules, also with chemico-physical changes and movement 

 of the pigment granules within the cell-bodies and of the cells themselves. 

 As is well known, the skin and subcutaneous tissues tend to become 

 pigmented when exposed to light and pigment granules or pigment cells 

 as a whole tend to move towards the source of light. This movement 

 has been demonstrated in the pigmented cells of the retina in both verte- 

 brates - and in vertebrates. •'• H. M. Bernard in 1896 suggested that eyes 

 first arose as local modifications of tissue induced by the crowding of 

 pigmented granules at spots in the skin which were most frequently 

 and brilliantly illuminated. In the Q.J. Micro. Sc. (1896-7), 39, 343, 



1 H. B. Adelmann (1928), Arch. Entzv. mech., 118. 



2 L. B. Airey, J. Comp. Neurol, and Psychol., 25, 1915 ; 30, 1918. S. R. 

 Detwiler, J. Comp. Neurol. & Psychol., 30, 1924. M. S. Mayou, Brit. J. Oph- 

 thalm., 1932, p. 227, and 1933, p. 477. 



3 H. M. Bernard, Q.J. Micro. Sc, 39, 43, 47. G. H. Parker, 1899, Bull. 

 Mus. Comp. Zool., 35, No. 6, p. 143. 



