THE PINEAL BODY 203 



constituting the caudalmost constituents of the pineal region, 

 are obviously specialized as neural mechanisms in the interest of 

 special sense receptors. The pars intercalaris posterior has, no 

 doubt, the same functional significance as the anterior inter- 

 calated area associated with the superior commissure. 



From a review of the several structures associated with the 

 epiphyseal complex in the pineal region, it is clear that the 

 majority of them when differentiated at all give rise to glandular 

 organs, while those which participate in neural mechanisms are 

 not only in the minority, but constitute a relatively small portion 

 of this area in the brain. Thus the paraphysis and paraphyseal 

 arch as a whole, the velum transversum, and the postvelar arch 

 are genetically glandiferous, while the superior commissure and 

 posterior commissure alone bear any apparent relation to neural 

 activity. 



In view of these facts, it would seem that whatever the func- 

 tions of the epiphyseal complex may be, the morphogenetic 

 impulse imparted to it from a region of the brain so preponder- 

 atingly glandiferious in its constituents could not fail to have 

 a profound influence upon the evolutional adaptation of the 

 epiphysis. Yet, in spite of the illumination which this genetic 

 association of the epiphyseal complex with definitely glandular 

 structures seems to shed upon its inherent tendencies in differ- 

 entiation, it must be in the intrinsic characters of the complex 

 itself that the solution of its problem is ultimately to be sought. 



2. Evidence based on the gross morphology of the epiphyseal 



complex 



a. Phyletic constancy. If such evidence as may be obtained 

 from the gross morphology of the epiphyseal complex is taken 

 into account, a number of reasons may be advanced to show 

 that it is quite impossible to conceive of the pineal body as a 

 vestigial structure. These reasons seem so cogent as to place 

 upon the arguments which would refute them an unusually 

 heavy burden. 



