360 THE PINEAL ORGAN 



the sides of the pit becoming elongated and transformed into lentigen, 

 or vitreogen cells, which secrete a clear, viscous fluid. This condenses 

 and forms a non-cellular or vitreous lens in the cavity of an ocellar pit 

 which is usually closed by the union of the margins of the fold over the 

 mouth of the pit and the formation of a cuticular and cellular lens super- 

 ficial to the vitreous lens (Fig. 248). 



Now in Apus and Branchipus Patten has demonstrated a very interesting 

 phase of the development of the triplacodal type of median or " Ento- 



Fig. 248. — Cross-section of Parietal Eye Vesicle of Apus. (After Patten.) 

 a. rt. : anterior retina. c.e.v. : common cavity of lateral and 



c. : chitinous plug closing the opening 



of the 

 exterior. 



common cavity to the 



c.e.v. 



parietal eye vesicles. 

 /. rt. : left retina. 

 pg. c. : large pigment cells bounding 



cavity of parietal eye vesicle. 



mostracan eye." The retinal placodes, which are four in number and 

 purely dermal in origin, are depressed so that they come to lie at the 

 bottom of separate pits, below the general surface of the surrounding 

 ectoderm. They approach the median plane and become infolded so 

 as to lie in the walls of a small common pit which opens into a larger 

 common chamber which is enclosed by the forward growth of a transverse 

 fold of the skin. This covers over both the median and the lateral eyes. 

 The narrow mouth of this common chamber, which appears as a small 

 pore in the skin, becomes closed by a chitinous plug (Fig. 248), and the 



