372 THE PINEAL ORGAN 



postero-lateral position to the postero-ventral position of the adult. 

 There is, however, a very important and significant change in the course 

 of the intracapsular fibres, which apparently shift from an attachment 

 at the outer ends of the retinal cells to the inner or deep ends of these cells, 

 which is the position they occupy in the adult. The migration of the 

 fibres takes place at the same time that the nuclei recede into the 

 deeper parts of the eye, and seems to be controlled by the growth of the 

 rhabdomeres. The exact process by which the change from the inverted 

 embryonic position of the retina to the final adult position takes place 

 are still obscure ; but it is quite clear that as a result of the involution of 

 the retinal area its morphologically deep surface, from which the nerve- 

 fibres primarily arise, becomes turned towards the light, and that its 

 originally superficial surface, in which the clear refractile rods become 

 developed in upright eyes, is in the early larval stages turned away from 

 the light. This apparently anomalous condition is changed during the 

 later stages of development, so that eventually the nerve-fibres are found 

 leaving the deep ends of the retinal cells, and in the adult animal the 

 rods or rhabdomeres are pre-nuclear and directed towards the light. 

 It is also certain that the retinas of both median and lateral eyes are strictly 

 hypodermal in origin and not neural. Owing to the manner in which 

 the involution takes place the median eye is of the three-layered, tri- 

 plostichous type, the superficial layer of the hypodermis or pre-retina 

 giving rise to the vitreous body and cuticular lens ; the middle layer 

 forming the inverted retina and the third or deep layer forming the sclera, 

 which becomes intimately fused with the retina. The retina contains 

 two kinds of cells, the visual or neuro-sensory cells and pigment cells. 

 Pigment is also contained in the neuro-sensory cells. The walls of these 

 cells develop pre-nuclear rhabdomeres, and a nerve-fibre emerges from 

 their deep ends. 



Parker draws the following conclusions with regard to the adaptive 

 structural changes which follow the inversion of the retina in the median 

 eyes of scorpions and spiders : " The striking similarity in the structure 

 and development of the median eyes of scorpions and the median eyes of 

 spiders has already been indicated. In both cases the retina by a process 

 of involution has become inverted. The question whether the retina was 

 functional during the involution of the eye was answered in the affirmative 

 by the phases noted in the development of the optic nerve. At least 

 the fact that the fibres of the optic nerve are at first attached to the morpho- 

 logical deep ends of the retinal cells, and only at a later date come to 

 emerge from the opposite end, is most easily explainable on the sup- 

 position that the retina was functional before involution. The primitive 

 eye would then consist of a single layer of retinal cells, from the deep 



