HYPOPHYSIS. 283 



SIGNIFICANCE OF THE HYPOPHYSIS CEREBRI. 



The pituitary body, or hypophysis, belongs to the series 

 of ductless "glands" (pineal body, thyroid gland, thy- 

 mus, etc.) which are such a characteristic feature of the 

 vertebrate organisation. It arises as an ectodermal invo- 

 lution from the roof of the stomodoeum, directed towards 

 the base of the primary fore-brain, from which the infun- 

 dibulum grows out. 



The pituitary involution becomes in most forms nipped 

 off from the stomodoeum, and then lies as a closed sac 

 in contiguity with the infundibulum. Later on it produces 

 a system of branches, the lumina of which tend to dis- 

 appear ; and in some forms {e.g. Mammalia) it undergoes 

 actual fusion with the infundibulum. 



The very constant relation of the hypophysis to the 

 infundibulum in the craniate Vertebrates (see Fig. 134) 

 naturally led to the supposition that there must originally 

 have been a functional connexion between the two struct- 

 ures of a similar nature to that which exists between the 

 olfactory pit and neuropore in Araphioxus. Recent re- 

 searches, however, have rendered it probable that such a 

 supposition is erroneous. Von Kupffer has discovered 

 the homologue of the lobus olfactorius of Amphioxus in 

 the craniate Vertebrates, and has shown that it occurs at 

 a point far removed from the infundibular region. 



Until recently it was also very generally thought that 

 the infundibulum represented the anterior end of the 

 brain, which had become bent downwards and backwards 

 by the cranial flexure. Kupffer, however, has brought for- 

 ward weighty reasons for doubting this view. According 

 to him, the infundibulum is essentially a downgrowth or 



