238 FREDERICK TILNEY AND LUTHER F. WARREN 



invertebrate pineal region of the brain can satisfy the require- 

 ments in this field of homology. Not only must such an area in 

 its general outlines be recognized, but the demonstration must 

 be given that every element entering into it has its homologue 

 in the vertebrate brain. For this reason it seems impossible at 

 present to accept any other view than that the median eye in 

 invertebrates and the parietal eye of vertebrates are analogous. 

 The supposition that they are homologues, however suggestive 

 and stimulating, can hardly be regarded, at present, as other 

 than speculative morphology. 



8. SUMMARY AND CONCLUSION 



I. The pineal region is preponderatingly glandiferous in its 

 derivatives. The morphogenetic impulse imparted by such 

 a gland-forming area could not fail to have a profound influence 

 upon one of its constituents, the epiphysis. 



II. a. The pineal body cannot be a vestige from the evidence 

 based upon its gross morphology, for the following reasons: , 



1. The phyletic constancy of the epiphysis in the vertebrate 

 phylum. 



2. Its variations and morphologic specializations. 



3. Its relatively greater phyletic constancy with reference to 

 other structures in the pineal region. 



4. The gross evidence of its progressive specialization in 

 ophidians, birds, and mammals. 



5. The increase in the epiphy so-cerebral index, from the 

 earliest stages to the latest periods of life in man. 



6. The resistance to the encroachment of a prominent neo- 

 morph in the mammalian brain, that is, the corpus callosum, 

 which has produced such marked alterations in the other con- 

 stituents of the diencephalic roof-plate. 



b. The pineal gland cannot be considered a vestige in the 

 light of the histological evidence, since the tendency toward 

 specialization is definitely in the interest of glandular formation 

 in ophidians, chelonians, birds, and mammals. Ontogenetically, 

 in two forms at least, in Felis domestica and man, the develop- 



