374 THE PINEAL ORGAN 



ends, which give rise to the rods and which were primarily directed 

 towards the surface, become deep ; whereas their inner ends, which 

 terminate in nerve-fibres, become superficial and lie in contact with the 

 cellular or vitreous lens which is developed as a modification of the 

 ordinary hypodermal cells where they cover the retinal layer. The 

 lower limb of the S-shaped bend or post-retinal layer, may become con- 

 densed so as to form a protective capsule (Fig. 251) or it may become 

 pigmented. Further, as described by Parker in the later stages of develop- 

 ment of the median eyes of scorpions (and spiders), the reversal of the 

 cell-elements caused by the infolding of the retina is corrected by migra- 

 tion of the nerve-fibres to the deep surface of the retina and the develop- 

 ment of secondary rods at the primarily inner ends of the cells, which 

 as a result of the inversion are now superficial. 



We shall see later that the same processes which occur in the develop- 

 ment of certain invertebrate eyes also take place in the evolution and 

 development of vertebrate eyes. With certain modifications as regards 

 details, there are to be noted a similar infolding of the ectoderm and 

 inclusion of the optic plates within the neural tube ; there is also a similar 

 differentiation of the retinal ectoderm into dioptric, receptive, and pig- 

 ment cells (Fig. 256 ; and Fig. 10, Chap. 3, p. 15). There is, moreover, the 

 same migration of cell-elements or their nuclei within the retina, and a 

 similar connection of the visual receptive cells with the cortex of the 

 brain by means of nerve ganglia and intervening plexiform bands or tracts 

 of nerve-fibres. 



In a work limited co the study of the morphology of the pineal organ 

 it would be inexpedient to do more than allude to the controversial 

 points which have been raised in the past by such authors as Patten and 

 Gaskell in relation to the important bearing which it has on the whole 

 problem of the evolution of vertebrates. But in this discussion on the 

 relation of the median and lateral eyes of vertebrates and invertebrates, 

 it is necessary that the reader should have a concise statement of these 

 views in order that he may judge for himself the principles that are con- 

 cerned in coming to a decision on this complex biological problem, and 

 which we shall endeavour to summarize in our concluding chapter. 



We propose in the first place to give a summary in his own words of 

 some of the more important views of Patten on the parietal eye of verte- 

 brates, which, as we have already stated, he regards as homologous with 

 the median eyes of the Entomostraca. 



1. "In vertebrates we recognize as belonging to the forebrain the 

 median or parietal eyes, the lateral eyes, and the olfactory organs. At 

 an early embryonic period they lie on the outer margins of the open 

 neural plate in similar positions to the ones they occupy in arthropods. 



