n8 



THE PINEAL ORGAN 



the surrounding hypoderm cells. The outer ends of the retinal cells are 

 clear and rod-like ; in the middle portion of each cell is the nucleus and 

 the inner end is continuous with a fibre of the optic nerve. 



According to the description given by Giinther in 1912 of the develop- 

 ment of the eyes of Dytiscus marginalis, a water-beetle, the young larva 

 has six ocelli on each side of the head and a rudimentary eye-spot. In 

 the early stage of development only ocelli are present ; at a later stage a 

 crescentic area lies in front of these, which is the rudiment of the com- 

 pound eye (Fig. 78). In section each ocellus is seen to be formed as a 

 slit-like depression of the epithelium. The cells at the base of the pit 

 end in visual rods ; each rod consists of two semicylindrical segments 

 adherent to each other and appearing to consist of a mass of agglutinated 



tSi vitr.c. 



p-ph.s. 



bas.c. 



Fig. 80. — Later Stage in the Development of the Compound Eye of Dytiscus 

 Marginalis, as seen in Vertical Section. (After Gunther.) 



fibrillar The cells lining the sides of the lower part of the pit bear 

 horizontally directed rods which are similar to the rods of the larger basal 

 cells, but are much smaller (Fig. 38, Chap. 3, p. 53). Nearer the mouth 

 of the pit are some cells which secrete a gelatinous, non-cellular vitreous 

 body. When the larva leaves the water and undergoes its pupal moult, 

 a fine, pigmented line surrounds each group of ocelli. This line, which is 

 horse-shoe shaped, afterwards thickens, and when the moult takes place 

 the lenses of the ocelli are torn away from the deeper pigmented portions, 

 since the lenses belong to the larval cuticle which is shed off from the 

 deeper pigmented parts. The horse-shoe pigmented portion has now 

 widened out and increased in breadth so as to form a crescent, the pig- 

 mented ocelli lying close behind its concave border. The cuticle covering 



