54 THE PINEAL ORGAN 



but also it is quite frequent to find a cuticular or corneal lens combined 

 with a vitreous lens formed from hypodermal cells in one and the same 

 eye, e.g. in the larva; of Dytiscus or Acilius (Figs. 38 and 13). 



In this connection it will be interesting to consider the capabilities of 

 lens-formation from the cutaneous or surface layer of ectoderm and 

 from the optic cup that have been demonstrated by experimental embryo- 

 logists ; and also the influence of light and darkness on the contraction 



IMLcct 



S.C— 'ffSS 



PS- 



-!-- y — \ cap. 



op.n. 



Fig. 39. — Schematic Section through a Dorsal Eye of Onchidium, showing 

 the Optic Nerve piercing the Retina, and its Fibres spreading out on 

 its Inner Surface as in the Lateral Eyes of Vertebrates. 



C. ep. : epithelium of cornea. op. n. : optic nerve. 



C. ct. : connective tissue layer of pg. I. : pigment layer of retina. 



cornea. r.b. : refractile body consisting of 

 /. cap. : fibrous capsule of eye. two very large cells. 



7i.f.l. : nerve-fibre layer of retina. s.c. : sensory cells of retina. 



or expansion of melanophores and on the movements of pigment granules 

 and pigment cells. Moreover, the multiplication of pigment cells that 

 takes place in fish reared on a dark background as contrasted with others 

 reared on a white background has a definite bearing on this subject. 



1 . Dependent differentiation of the lens vesicle. — An example of this is 

 furnished by the differentiation of the lens from epidermis, which is depen- 

 dent on the influence of the optic cup (Fig. 40). * See p. 174, Chap. 15. 



1 O. Mangold (1929), Arch. Entw. mech., 117. 



