550 



THE HUMAN EMBRYO. 



At its first formation (Fig. 237) the pharynx is of approxi- 

 mately uniform width along its whole length ; but at an early 

 stage the anterior part widens very greatly and the whole 

 pharynx becomes funnel-shaped, with the apex directed back- 

 wards (Figs. 238 and 239). 



The condition of the pharynx on the fifteenth day is shown 

 in horizontal section in Fig. 237, which should be compared 



with Figs. 197 and 232, 

 A! which represent the same 

 embryo in surface view, 

 and in sagittal section. 

 The visceral arches are 

 seen to form prominent 

 ridges projecting into the 

 pharynx, and separated 

 from one another by 

 grooves, the visceral 

 pouches. Of the visceral 

 arches, the mandibular, 

 MN, and hyoidean, HY, are 

 well developed ; and be- 

 hind these the first and 

 second branchial arches, 

 BR 1 and BR 2 , are recognis- 

 able, though less clearly 

 defined. 



The hyomandibular and first branchial pouches are well 

 formed ; and corresponding to them on the outer surface of the 

 pharynx are well-marked external visceral grooves, clearly seen 

 in surface views of the embryo (Fig. 197, HM, HC 1 ). The corre- 

 sponding visceral pouches and grooves, on the inner and outer 

 surfaces of the pharynx respectively, do not quite meet, but are 

 separated by thin membranous partitions, of which the most 

 anterior one, EB, between the mandibular and hyoidean arches, 

 becomes ultimately the tympanic membrane. 



Further back, there are less strongly marked second bran- 

 chial, and third branchial pouches or grooves on the inner surface 

 of the pharynx, with slight indications of corresponding visceral 

 grooves on the outer surface. 



Towards the end of the third week, and in the early part of 



FIG. 237. The floor of the pharynx of a 

 Human Embryo fifteen days old, seen 



gf from above. (Cf. Figs. 197 and 232.) 

 (From His.) x 50. 



Al, first aortic arch, in the mandibular 

 arch. A2, second aortic arch, in the hyoid 

 arch. BB.1, first branchial arch. BR2, 

 second branchial arch. C, body cavity, or 

 coslom. EB, membrane closing the hyoman- 

 dibular cleft, which becomes afterwards the 

 tympanic membrane. FL, furcnla. HY, 

 hyoid arch. MN, mandibular arch. TTJ, 

 tuberculum impar. 



