278 THE PINEAL ORGAN 



I. The Parietal Eye. 



1. The parietal eye of Iguana, arising from the constriction at the 

 distal part of the primary epiphyseal evagination, appears on the 9th 

 day as an oval vesicle which is more or less constricted off from the proximal 

 epiphysis. 



2. The originally markedly biconvex lens in the course of develop- 

 ment takes on a flat, slightly biconvex form or even a plano-convex 

 form. 



3. From the 14th to 18th days the entire inner side of the eye cavity 

 is covered with cilia. A strong nerve unites the under side of the eye 

 vesicle with the roof of the diencephalon. The retina shows two zones, 

 one inner without cells and an outer cellular zone. Black pigment begins 

 to appear on the retinal cells, and from the surrounding mesoderm a 

 connective tissue capsule is formed which envelops the nerve and eye. 



4. In 24 to 26 days the parietal nerve has reached its highest degree 

 of development. Through the inward growth of the nerve-fibres the 

 retina is divided into an outer and inner cell layer, between which the 

 subdivisions of the nerve can be seen as a nerve-fibre layer. In the inner 

 zone an abundance of pigment has been formed. 



5. In 35 to 40 days the nerve and nerve-fibre layer shows significant 

 signs of degeneration. The amount of pigment has greatly increased. 



6. In the adult Iguana the pineal eye shows the varied features which 

 are characteristic of rudimentary organs. 



7. The nervous elements seem to have entirely disappeared in the 

 retina, and the formation of pigment has become so excessive that a 

 recognition of the structural relations is often impossible. 



II. The Parietal Nerve. 



1. The nerve which enters the pineal eye is dissimilar in its develop- 

 ment to that of the optic nerves of the paired eyes. 



2. At the end of the 9th day there is still no trace of a nerve to be seen 

 entering the vesicle, which has already separated from the pineal. 



3. By the 14th day the nerve is formed, and passes from the floor of 

 the eye vesicle to a cell accumulation lying in the roof of the diencephalon, 

 called the centrum parietale. 



4. The parietal centre lies asymmetrically to the right of the middle 

 line, immediately in front of the pineal evagination. 



5. In 24 to 27 days the nerve is surrounded by a connective-tissue 

 sheath (perineurium), and the parietal centre now lies in a direct line 

 with the right ganglion habenulae. 



6. In 35 to 40 days the nerve shows undoubted signs of degeneration 

 that already appear to have produced atrophy of its central part. 



