202 



COMPARATIVE ANATOMY 



outgrowths of the roof; and finally, the infundibulum, formed as an 

 extension of the floor, together with part of the pituitary body 

 (hypophysis). Another portion of the pituitary body is derived from 

 the epithelium of the primary oral involution (stomodseum). 1 



The pineal apparatus consists of the epiphysis or pineal organ 

 proper, which persists in a more or less vestigial condition in 

 all Vertebrates, and of a more anterior outgrowth which may be 

 called the parietal organ, arising from the epiphysis or inde- 

 pendently from the roof of the diencephalon, and becoming 

 atrophied in the majority of Vertebrates. Each of these structures 



n 



FIG. 151. DIAGRAM ILLUSTRATING THE STRUCTURE OF THE HYPOPHYSIS OF 

 VERTEBRATES. A, PETROMYZON ; B, PISCES ; (7. AMPHIBIA ; D, SAUROP- 

 SIDA; E, MAMMALIA. (After Sterzi.) 



H, " chromophilous " portion, and H 1 , " chromophobic " portion of hypophysis; 

 P, infundibular process ; $, saccus vasculosus. 



represents a vestigial sensory organ, and may retain to a greater 

 or less extent the characters of a median eye, which in some cases 

 has probably a light-perceiving function. Certain facts indicate 

 that these organs may have been paired primitively or that the 

 two correspond to members of a pair ; but further researches are 

 desirable on this point, as well as on the relation of the two organs 



1 Possibly the endodermic epithelium of the primary fore-gut may also take 

 part in its formation. 



