THE CEANIAL BASIS. 



737 



The flask-like outgrowth of the buccal epiblast which gives rise to the hypo- 

 physis cerebri, is now gradually shut off from the corneous layer and cavity of the 

 mouth, iirst by the constriction, and subsequently by the closure of its place of 

 communication. There remains however, for a considerable time, a longish. 



Fig. 537. 



Fig. 538. 



Fig. 537. — OcTLiXE Plan 

 ViKW OF THE Upper 

 Part of the Body of an 

 Embryo Pig, two-thirds 



OF AN INCH IN LeNGTH. 



Magnified seven dia- 

 meters (from Parker). 



Fig. 538. — Plan op the 

 Skull, &c., of the same 

 Ejibryo seen from below. 

 Magnified ten diameters 

 (from Parker). 



In this a7id the preceding 

 figure the letters, where j)re- 

 sent, indicate the following 

 parts : — 



c' to 0^ , the five primary 

 divisions of the brain ; a, the 

 eye ; n, the nose ; m, the 

 mouth ; <?•, cartilage of the 

 trabeculEe ; ctr, cornua tra- 

 becularum ; pn, prenasal car- 

 tilage ; 2}p[/> pterygo-palatine 

 cartilage ; mn, the mandi- 

 bular arch with Meckel's car- 

 tilage ; te, first visceral cleft 

 which becomes the tympano- 

 eustachian passage ; au, the 

 auditory vesicle ; hi/, the 

 cerato-hyoid arch ; Ir, the 

 branchial bars and clefts, 

 1 to 4; tItJi, the thyro-hyoid ; 

 py, the pituitary fossa ; ch, 

 the notochord in the cranial 

 basis, .surrounded by the in- 

 vesting mass (ii') ; vii, facial 

 nerve ; ix, glosso-pharyngeal ; 

 X, pneumog;istric ; xii, hypo- 

 glossal nerve. 



thread of union between 

 the two (fig. 535, C, 7/). 

 The epithelium of the en- 

 closed portion subsequently 

 undergoes development 

 into glandular coeca and 

 cell-cords, and its internal 

 cavity becomes gradually 

 obliterated. This fonns 

 the anterior part or lobe 

 of the pituitary body. The 

 posterior part owes its origin to the combination with mesoblastic tissue of a. 

 widened extension of the infundibular process of the brain, which is thrust in 

 between the sac of the pituitary body and the dorsum sella;. The nervous struc- 

 ture of this posterior lobe afterwards disappears in the higher animals, but in 

 the lower it retains its place as a part of the brain. 



vol.. ir. 3 B 



