52 



THE PINEAL ORGAN 



of the " neural plate." The distinction is also of importance with 

 reference to the comparison of the type of lens found in the pineal eye of 

 vertebrates with the forms of lens found in invertebrates, whether in the 



Fig. 37. 

 A — Two isolated retinula; from the compound eye of a prawn, Palcemon squill a. 



cf.: 

 Sc. , 

 cc . . 

 CC. 



(After Grenacher.) 

 rl. 



n . 



bm. : 

 Op. n. 

 Rm. : 



retinula. 



nuclei of retinular cells. 



basement membrane. 



: optic nerve. 



rhabdome. 



corneal facet. 

 Semper 's cells, 

 outer crystalline cone. 

 : middle crystalline cone. 

 cc" . : inner crystalline cone (hollow) 

 pg. pg'. : pigment. 



B — An isolated " crystal cone." 



C — Transverse section of a retinula about its middle, bleached. 



D — Transverse section of the posterior or inner end of the retinula. 



median or lateral eyes. It was maintained by some authors that the 

 existence of a corneal or cuticular type of lens in the central eyes of certain 

 arthropods and other classes of invertebrates put completely out of court 

 any comparison of the " pineal eye " of vertebrates with the central eyes 



