436 



PITUITARY BODY. 



is dilated terminally ; while the passage connecting it with the 

 mouth becomes narrower and narrower, and is finally reduced to 

 a solid cord, which in its turn disappears. 



Before the connection between the pituitary vesicle and the 

 mouth is obliterated the cartilaginous cranium becomes developed, 

 and it may then be seen that the infundibulum projects through 

 the pituitary space to come into close juxtaposition with the 

 pituitary body. 



After the pituitary vesicle has lost its connection with the 

 mouth it lies just in front of the infundibulum (figs. 250 and 

 255 Jiph and fig. 254 pf) ; and soon becomes surrounded by 

 vascular mesoblast, which grows in 

 and divides it into a number of 

 branching tubes. In many forms 

 the cavity of the vesicle completely 

 disappears, and the branches become 

 for the most part solid [Cyclostomata 

 and some Mammalia (the rabbit), 

 Elasmobranchii, Teleostei and Am- 

 phibia]. In Reptilia, Aves and most 

 Mammalia the lumen of the organ is 

 more or less retained (W. Muller, No. 

 344). 



Although in the majority of the 

 Vertebrata there is a close connection 

 between the pituitary body and the 

 infundibulum, there is no actual fusion 

 between the two. In Mammalia the 

 case is different. The part of the in- 

 fundibulum which lies at the hinder 

 end of the pituitary body is at first a 

 simple finger-like process of the brain 

 (fig. 255 inf], but its end becomes 

 swollen, and the lumen in this part 

 becomes obliterated. Its cells, origi- 

 nally similar to those of the other 

 parts of the nervous system and even 

 (Kolliker) containing differentiated nerve-fibres, partly atrophy, 

 and partly assume an indifferent form, while at the same time 



FIG. 256. TRANSVERSE SEC- 

 TION THROUGH THE FRONT PART 

 OF THE HEAD OF A YOUNG PR1S- 

 TIURUS EMBRYO. 



The section, owing to the cran- 

 ial flexure, cuts both the fore- and 

 the hind-brain. It shews the pre- 

 mandibular and mandibular head 

 cavities ipp and ipp, etc. The 

 section is moreover somewhat 

 oblique from side to side. 



fb. fore-brain; /. lens of eye; 

 ;. mouth ;//. upper end of mouth, 

 forming pituitary involution; \ao. 

 mandibular aortic arch ; ipp. and 

 ipp. first and second head cavities ; 

 \vc. first visceral cleft; V. fifth 

 nerve; aim. auditory nerve; VII. 

 seventh nerve; aa. roots of dorsal 

 aorta ; acv. anterior cardinal vein ; 

 ch. notochord. 



