156 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



the middle, forming a lens, while the cells of the proximal side elon- 

 gate, each becoming differentiated into a distal, rod-like end and a 

 proximal portion which contains the nucleus and is connected with a 

 nerve fibre. Pigment is deposited between these cells so that the 

 whole forms a retina. An important point, to be better appreciated 

 after the consideration of the paired eyes, is the fact that these pari- 

 etal eyes are Uke those of most invertebrates in having no inversion of 

 the retina. How far these eyes are actually functional is not settled. 

 Even in Sphenodon, where it is best developed, experiments have 

 resulted in no decided reactions. 



In other vertebrates the parietal organ does not pass outside the 

 skull, and even may not appear transitorily in development. The 



Fig. 162. — Parietal eye of Anguisfragilis, after Nowikoff. ct, connective-tissue cells 

 around nerve; gc, ganglion cells; /, lens; n, nerve fibres; pn, parietal nerve; pc, pigment 

 cells; r, retinal cells; vb, vitreous body. 



pinealis to some extent may take its place and often shows, as in 

 certain elasmobranchs and lizards, traces of a visual structure. In 

 the anura its tip approaches the skin and later is cut off from the brain 

 by the development of the skull, forming the so-called frontal organ 

 or 'brow spot,' visible from the exterior. Pineal and parietal 

 organs differ in their nerve supply, the parietal being connected with 

 the superior commissure of the 'twixt-brain, the pinealis and its de- 

 rivatives with the posterior commissure. In the higher vertebrates 

 the epiphysial structures are completely covered by the backward 

 growth of the cerebrum. The large parietal foramina in many ex- 

 tinct reptiles and stegocephals (figs. 92, 99, 100, 103) would seem to 



