366 THE PINEAL ORGAN 



Now in comparing the compound lateral eyes with the simple eyes of 

 the larva, it may be presumed that each ommatidium of the compound 

 eye represents an ocellus and that the whole compound eye is formed 

 by an aggregate of modified ocelli. The process of pit formation by a 

 downgrowth of epithelial cells, however, appears to have been abbre- 

 viated in the ontogenetic development of the ommatidia, and although a 

 potential lumen may be considered to exist in the central axis of each 

 ommatidium, there is no actual cavity containing a vitreous mass, as in 

 the simple ocellus (Fig. 38, Chap. 3, p. 53). There is nevertheless a 

 distinct resemblance between the single basal cell of the rhabdome sur- 

 rounded by the six peripheral cells of each retinula of the compound eye 

 and the two large visual cells, with the adjacent long, slender cells sur- 

 rounding it, which lie at the bottom of the pit of the simple ocellus. The 

 probability of compound eyes being formed by an aggregation of simple 

 ocelli is, moreover, supported by intermediate conditions between com- 

 pound and simple eyes, such as are seen in the lateral and central eyes of 

 Euscorpius (Figs. 94, 95, Chap. 11, pp. 132, 133) and Limulus (Figs. 86, 87. 

 Chap. 11, pp. 124, 125), in both of which there is a combination of retinular 

 or rhabdome formation with a continuous corneal surface, suggesting 

 fusion of phylogenetically separate ocelli. The schizochroal eyes of 

 Phacops, a Trilobite of the Devonian period, also support the view that 

 the compound lateral eyes arise phylogenetically by fusion of adjacent 

 ocelli into a single optic plate, for in Phacops the ommatidia are definitely 

 isolated and separated by interstitial areas of the test (Fig. 98, Chap. 11, 

 p. 137), the corneal facets thus being discontinuous, as contrasted with 

 the more common continuous corneal surface of the holochroal type. 



The various stages by which the simple upright eyes of the lower 

 types of arthropods appear to have been transformed in the course of 

 phylogeny into the paired compound eyes of the higher types has been 

 studied by Lankester, Parker, Patten, Watase, and others, who have also 

 endeavoured to fill in and explain certain intermediate stages between 

 the larval and adult forms, which are often absent in those arthropods 

 in which there is a definite metamorphosis in the life history of the animal, 

 as for instance in many of the Insecta. In tracing these changes, it will 

 be necessary first to review briefly some of the causes of these alterations 

 in form and structure. 



Starting with the assumption that the effect of light on protoplasm is 

 to produce a chemical change that results in the formation of pigment, 

 and further that this process renders the cell at once capable of absorbing 

 light and reacting to it, we may note next that this reaction is a chemical 

 change which is accompanied by decomposition of a part of the protoplasm 

 of the cell during the manufacture of the pigment, and further that the 



