424 HISTORY OF THE HUMAN BODY 



elaborate method of development, involving as it does two dis- 

 tinct elements, is disproportional. It is thus generally sup- 

 posed that we have a vestigial organ like those developing dor- 

 sally and laterally from the same region, and that it, like them, 

 represents the remnant of an organ of considerable importance 

 in some unknown ancestral group. This organ is a noticeable 

 feature of the ventral aspect of all vertebrate brains, and bears 

 the noncommittal name of hypophysis, literally that which 

 grows beneath, in allusion to its position. In most skulls, es- 

 pecially in the more completely ossified one of the amniotes, 

 there is a distinct depression for its lodgment (the sella turcica 

 of human anatomy), and, as the hypophysis is often connected 

 with the brain by a narrow stalk around which the bone may 

 fit quite tightly, it is seldom removed in its entirety with the 

 brain, and hence its true relations are apt not to be wholly 

 understood. 



The portion contributed by the diencephalon is in the form 

 of a hollow cone or funnel, the infundibulum. About this the 

 invagination from the mouth cavity, which is glandular in its 

 nature, and termed pituitary body, becomes developed, and by 

 the secondary loss of the original connection between this latter 

 and the roof of the mouth, through the development of the 

 palate, the hypophysis is made to appear like a simple organ, 

 attached to the brain. 



Although there is no feeling of certainty among morpholo- 

 gists concerning the original form of this organ, the opening 

 of the pituitary portion into, or rather from, the exterior 

 in the more primitive forms, suggests that this part may repre- 

 sent the rudiment of an earlier mouth, the palaostoma, with 

 which, as shown by other data, the prevertebrate ancestors 

 seem to have been equipped prior to the development of the 

 definite vertebrate mouth, the neostoma* 



Aside from these diverticula and the organs found in asso- 



*The pituitary diverticulum arises in gnathostomes from the ectoderm 

 of the stomatodaeal invagination, but in cyclostomes is beyond the limits 

 of the mouth and pushes in from the external surface of the head in close 

 association with the medial nasal invagination. For farther details and 

 theories concerning this part see Chapters VI, XI and XII ; also Fig. 129. 



