402 Nose, Pharynx and its Derivatives in Man 



sembles the one just preceding in general outline except that the angles 

 are decidedly rounded. The 1st visceral pouch appears as a prominent 

 ridge (sulcus tubo-tympanicus) running outwardly and dorsal ly and 

 ending in a rounded free prominence. The rounded fossa just back 

 of it in embryo CLXIII has become here a flattened surface. Seen 

 from the inside (caudal surface) there is a large central rounded cavity 

 occupied by the tongue. A prominent U-shaped ridge separates it 

 from a similar shaped groove, marking the position of the mandible. 

 At the dorsal end of the tongue cavity a small conical projection marks 

 the position of the thyreoglossal duct. Directly dorsal from that the 

 larynx arises and the cavity shows as a T-shaped body. The top of 

 the T facing ventrally. Caudalward the shape of the cavity changes 

 gradually until the trachea is reached, when it has assumed the shape 

 already described. Seen from the front (ventrally) the nasal cavities 

 project outward from a large elevated fold in the middle line. They 

 project away from the middle line at an angle of about 20°. 



Emhrijo CLXXV. 



This embryo has a length of neck-breach 13 mm., vertex-breach 

 13 mm. It is an embryo of the early part of the 6th week. This 

 model also includes the mucous membrane and with a few interesting 

 exceptions very much resembles the one just described. Looked at 

 from the side the 1st visceral pouch has become somewhat complicated 

 and altered. The most noticeable change being a prominent fold run- 

 ning from the 1st visceral pouch ventrally. The most striking differ- 

 ence noted from the side view is the lack of symmetry of the two sides. 

 On the right side the hollow stalk connecting the lateral thyroid rudi- 

 ment to the pharynx still persists. On the left side it has separated 

 cojnpletely and the enlarged double end is fused with the now enlarged 

 horseshoe-shaped median thyroid rudiment. On the right side the 

 enlarged end of the lateral rudiment is in contact with the median 

 rudiment but has not fused with it. Seen from above, this model differs 

 from that of CIX chiefly in the increased rounding of the angles. The 

 1st visceral pouch, however, has changed and the fold running ventrally 

 is much more prominent. This stage resembles the one figured by 

 Piersol in Fig. 13 of his article already referred to. The hypophysis 

 has just separated, although a slight thickening of epithelial elements 

 mark its position on the model. 



Seen from below (the inside) the model presents an even rounded 

 hollow with the horseshoe-shaped groove surrounding it. This horse- 



