216 FREDERICK TILNEY AND LUTHER F. WARREN 



/. Resistance to the encroachment of the corpus callosum. An- 

 other characteristic in the ontogenesis of the epiphysis, especially 

 in mammals, speaks against the possibility of its being a vestige 

 and indicates in it a tenacity as a morphologic structure so 

 marked as to suggest the probability of some inherent functional 

 activity. With the advent of the commissural fibers whose mas- 

 sive collection goes to make up the corpus callosum, the mam- 

 malian brain takes on a character not observed in the lower 

 forms. The gradual extent of this great 'interhemispheral com- 

 missure in a caudal direction subjects the entire roof of the 

 diencephalon to new conditions. The influence of these new 

 conditions is readily seen in the flattening of the dorsal sac and 

 the reduction of the paraphyseal arch. Yet, even in the in- 

 stances in which the corpus callosum extends far enough caudad 

 to reach the midbrain, the epiphysis withstands its. encroach- 

 ment and gives evidence of a resistive adaptation against the 

 pressure of the new structure. It seems fair to presume that if 

 there were vested in the pineal body an inherent tendency to 

 retrograde, under the pressure of this newly developed mam- 

 malian structure which has so uniformly altered the configura- 

 tion of other elements in the diencephalic roof-plate, the epiphysis 

 itself must have given evidence of much less resistance or per- 

 haps have Succumbed altogether. Its evident effort at adapta- 

 tion has already been referred to in the classification of the 

 epiphysis in mammals which, according to Cutore, 76 shows a 

 disposition on the part of the organ to accommodate itself to 

 the presence of the corpus callosum, in some forms being retro- 

 callosal in position, in others supracallosal, and still again main- 

 taining itself in all its morphologic intactness in a distinctly 

 subcallosal position. 



If the epiphysis is to be considered a vestige, in view of the 

 morphologic evidence above summarized, it seems apparent 

 that the burden of proof rests with those making the claim that 

 it is a rudimentary structure. To maintain this position they 

 must meet with some well-sustained objections the following 

 established facts: 



