986 EVOLUTIONAL MODIFICATIONS 



process, than which tlicrc is no inoic decisive example in the whole realm 

 of organized matter. 



E\'OLUTioNAL Significance of the Epiphysis Cerebri ix the 

 Primates. Another side of the collieular deUoresceiice o{ the mesen- 

 cephalon may perhaps be touched upon somewhat tentatively, l)ut not 

 without a sense oi growing conlidence as time de\ elops more ample 

 understanding of the important structures known as the endocrine glands. 

 The cei^halic extremity of the longitudinal mtercollicular sulcus in the 

 j:)rimate series tends to become broader and Hatter, forming a comparatively 

 roomy fo.ssa at the cephalic extremity of the midbrain. In this chpression 

 rests a specialization of the diencejjhalic roolplate, the c]>ii>hysis 

 cen'hri. This structure has a long and complicatt'd history Irom the beginning 

 of vertebrate sjjccialization. In its earliest representation it makes itsell 

 apparent in a highly specialized form and gi\es rise to a pair ol eyes situated 

 in the middle of the forehead. This is the primordial condition of the epiphysis 

 cerebri in the CNclostomes. After man\ lluctiiations in passing through the 

 selachians, ganoids and teleosts, in \\hicli it manifests a general retrogres- 

 sion from its sensory specialization, it shows some tendency to develop m a 

 glandular direction. Then it appears in the amphibians once more as a highly 

 different iated distance rece])tor, apparentlx for the pi'rception ol light. In 

 some of the lower rt-[)tiles it becomes even more highly specialized as, lor 

 example, in the parietal eye ol sphenodon. In the bird it takes lorm as a 

 complexly organized gland whose special ducts distribute its secretion 

 directly into the cavity of the third \cntrich'. In the mammals, howe\er, its 

 development is otherwise. Onl\ in the t'arliest stages ol ontogenesis does it 

 show tendencies toward glandular dilierent lation, and therealter undergoes 

 degencratixe changes. In man, in later life, it gixes exidence ol ad\anced 

 degenerati\'e change characterized In the deposition in it ol a sand-like sub- 

 stance, know n as aeervulus cerebralis. In early stages ol human de\ elopmeiit, 



