NERVOUS SYSTEM OF THE VERTEBRATA. 



359 



Fig. 25G. 



THKOUOH THE 



SECTION 

 OF THE 



is obliterated the cartilaginous cranium becomes developed, and it 

 may then be seen that the infundibulum projects through the pitui- 

 tary 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 liph and 

 fig. 254 pt); and soon becomes sur- 

 rounded 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. Midler, 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 

 (Kolhker) containing differentiated 

 nerve-fibres, partly atrophy, and partly 

 assume an indifferent form, while at 

 the same time there grow in amongst 

 them numerous vascular and connec- 

 tive-tissue elements. The process of the infundibulum thus meta- 

 morphosed becomes inseparably connected with the true pituitary 

 body, of which it is usually described as the posterior lobe. The 

 part of the infundibulum which undergoes this change is very pro- 

 bably homologous with the saccus vasculosus of Fishes. 



The true nature of the pituitary body has not yet been made out. It is 

 clearly a rudhueHtary organ in existing craniate Vertebrates, and its 

 development indicates that when functional it was probably a sense organ 

 opening into the mouth, its blind end reaching to the base of the brain. No 

 similar organ has as yet been found in Amphioxus, but it seems pos.<ible 

 ))erhaps to identify it with the peculiar ciliated sack placed at the opening 



Transverse 

 front part 



HE.iD OF A YOUNG PrISTIURUS EMBRYO. 



The section, owing to the cranial 

 flexure, cuts both the fore- and the 

 hiud-brain. It shews the preman- 

 dibular aud mandibular head cavities 

 1^;;; and Ipp, etc. The section is 

 moreover somewhat oblique from ^ide 

 to side. 



fh. fore-brain ; I. lens of eye ; m. 

 mouth ; pt. upper end of mouth, 

 forming pituitary involution ; lao. 

 mandibular aortic arch ; \pp. and 

 2pp. first and second head cavities ; 

 Ivc. first visceral cleft ; V, fifth 

 nerve; aim. auditory nerve; IT/, 

 seventh nerve; aa. roots of dorsal 

 aorta; acv. anterior cardinal vem; 

 ch. notochord. 



