400 Nose, Pharynx and its Derivatives in Man 



visceral pouch is crescentic in outline with a part projecting ventrally as 

 shown in Fig. 5. Seen from the inside (caudal surface) the model shows 

 the two ridges running from the prominences of the 1st visceral pocket 

 toward the middle line. They are not so pronounced in the model of 

 embryo II and are less prominent in the model of this embryo (CLXIII) 

 that includes the mucous luembrane than in the one where only the 

 cavity is modeled. At the point of Junction of these ridges in the 

 middle line a small cylindrical projection marks the remnant of the 

 thyreoglossal duct. The median thyroid rudiment has separated com- 

 pletely and sunken to the level of the 3rd visceral cleft. In front of 

 these ridges the tuberculum impar has made a large depression. The 

 median ridge seen in the model of embryo II has disappeared. Behind 

 these ridges are two depressions showing the position of the two dorsal 

 tongue rudiments. Behind (dorsalward) is still another depression 

 situated in the median line the significance of which is not clear. Two 

 ridges run caudalward and unite in the middle line to form a flattened 

 body. This is the rudiment of the larynx. 



Emhrijo CIX. 



This embryo has a neck-breach length of 10.5 mm. and a vertex- 

 breach of 11 mm. It is about 33 days old and so would be an embryo 

 of the latter part of the fifth week. In spite of the apparently short 

 time between this stage and the one just described very decided changes 

 have occurred. The model includes the mucous membrane and is rela- 

 tively large. In this model the angles are not so sharp and the various 

 structures show a tendency to become rounded. Seen from the side 

 as in Fig. 7 the same prominent bends are recognized but they are 

 not so pronounced as in the model of embryo CLXIII. The ventral 

 l>end is larger and is joined to the nasal cavity as the illustration shows. 

 The hypophysis is relatively higher and nearer the angle of the bend. 

 The nasal cavities join the oral cavity at its edge and the posterior nares 

 are in a position similar to the permanent condition in the frog. The 

 ])rominence of the 1st visceral pocket is more complicated and the ridge 

 running in from it toward the median line noted in the other two 

 models is somewhat broken and irregular. The 2nd visceral pouch 

 shows as a small knob projecting caudalward. The 3rd visceral pouch 

 has disappeared as such and the 4th is prolonged into a hollow tube 

 with two knobs on its caudal end. These will be described later as . 

 the lateral rudiments of the thyroid. At the point where they are 

 attached the pharynx abruptly constricts into the oesophagus. The 



