4 oo COMPARATIVE ZOOLOGY OF CHORDATES 



from the superficial ectoderm of the front of the head, and the 

 infundibulum which is a down-growth from the floor of the 

 forebrain. In Myxine these two constituents remain separated 

 by connective tissue, but in all the remaining animals they are 

 intimately connected and fused. In the Tetrapods it is possible 

 to distinguish four parts in the pituitary, of which three (the 

 anterior, intermedia, and tuberalis) arise from the hypophysis, 

 and one (the nervosa) arises from the infundibulum. The 

 intermedia is always (except in Myxine) plastered on to the 

 nervosa, and the two together form the neuro-intermediate 

 lobe. This is separated from an anterior lobe (formed of the 

 anterior part) by the hypophysial cleft which represents the 

 original cavity of the hypophysial ingrowth, Rathke's pocket. 



Fig. 174. — The pituitary body of a cat, seen, A, from the left side ; B, in 

 longitudinal section. 

 al, anterior lobe ; he, hypophysial cleft ; ic, infundibular cavity ; nil, 

 neuro-intermediate lobe ; pa, pars anterior ; pi, pars intermedia ; pn, pars 

 nervosa ; pt, pars tuberalis ; tc, floor of the brain. 



In some animals, the hypophysial cleft becomes obliterated 

 in the adult. 



In evolution, the hypophysis appeared before the infundi- 

 bulum, for in Amphioxus the latter is not represented, whereas 

 the hypophysis is present in the form of the preoral pit. The 

 preoral pit communicates with the (left) anterior head-cavity 

 just as the hypophysis communicates with the premandibular 

 somite in a number of Craniates. In the adult Amphioxus 

 the preoral pit becomes absorbed in the oral hood and gives 

 rise to the ciliated organ which produces a current of water 

 towards the mouth. Thereafter it probably sank into the 

 tissues and became a gland secreting by a duct into the mouth. 

 This duct (which represents the open mouth of the cavity of 



