THE SENSE ORGANS 593 



side to form a biconvex lens, which concentrates light upon the retinal 

 layer below. Three kinds of cells are distinguishable in this retinal layer, 

 sensory flagellated cells, pigment cells alternating with the sensory cells, 

 and ganglion cells connected with the brain by the parietal nerve. Except 

 in the region of the lens, the entire vesicle is surrounded by a single layer 

 of pigment epithelium. The lumen of the vesicle is filled with translucent 

 processes of the surrounding epithelial cells, and thus is analogous with the 

 vitreous body of the lateral eyes. 



In mammals a parietal organ is wanting. The pineal organ is glan- 

 dular. Its function is problematic, but is commonly assumed to be 



Fig. 491. — Section through brain of a Squalus embryo, 15 mm. long, showing the 

 early appearance of two epiphysial structures, the parietal organ {p) which is already 

 degenerating, and the pinealis (e) ; h, hypophysis, growing in from the oral epithelium; 

 i, inf undibulum ; n, notochord; v, velum transversum. (From Kingsley's " Comparative 

 Anatomy of Vertebrates.") 



endocrinal. The rich vascular supply of the organ suggests this, but 

 experiments to demonstrate the secretion of hormones have failed, and 

 the removal of the gland does not cause death or affect health. 



DEVELOPMENT OF THE PINEAL ORGAN IN MAN 



The pineal organ arises in a four-months embryo as a hollow tubular 

 outgrowth from the roof of the diencephalon. By the formation of 

 secondary lateral outgrowths, the anlage is converted into a compound 

 vesiculated structure richly supplied with blood vessels. The vesicular 

 structure later disappears, and the adult organ consists of solid masses of 

 polygonal epithelial cells of which various sorts may be distinguished by 

 their different staining properties. No ganglionic or nervous elements 

 can be recognized. Mulberry-like concretions sometimes occur. 



