39i Nose, Pharynx and its Derivatives in Man 



not exactly symmetrical as the right is caudal to the left one. Behind 

 these distinct prominences two pairs of waves can be discovered in the 

 outline of the edge of this flattened surface. These are the first traces 

 of the developing 3rd and ith visceral pouches. In the figure they 

 have been almost completely lost. This surface is curved convexly 

 between the prominences of the 3nd pair of visceral pouches and gradu- 

 ally flattens oralwards and caudalwards. Viewed from the side (Fig. 1) 

 the same structures as noted from above, as well as some others on the 

 caudal side of the plate, are visible. Directly under the prominence 

 representing Seesel's pocket there is another of similar shape, but 

 smaller, for the mouth. Just in front (ventral) of this the stomodseum 

 appears as a decided pouch or pit. 



On the under side of the model, on each side a ridge runs toward 

 the middle line from the first pair of visceral pouches. At their point 

 of junction in the middle line the median thyroid rudiment shows as 

 a rounded eminence. This rudiment looked at directly from below is 

 broader laterally than it is dorsoventrally. 



The lack of bilateral symmetry of the pharynx and of the oesophagus 

 is very marked. The position of the 2nd pair of visceral pouches has 

 already been mentioned; in addition the left side of the oesophagus is 

 thicker and heavier. At its caudal end it curves ventrally and ends in 

 an eminence representing the rudiment of the liver. On the right side 

 the ojsophagus comes to a rounded edge. A cross-section would be 

 wedge-shaped with the base of the wedge toward the left and the apex 

 toward the right. 



Embryo II. 



This embryo has a vertex-breach length of 6 mm. and a neck-breach 

 length of 7 mm. It is about 24 days old or, in round numbers, it is 

 ,an embryo of the 4th week. The model of this region shows that 

 marked changes occur in the period intervening between this and the 

 preceding stage. The various regions are easily recognized. Seen 

 from above this model is nearly an isosceles triangle. The base repre- 

 sents the mouth and hypophysis and the rounded apex the dorsal wall 

 of the oesophagus. The sides of the triangle show three rounded 

 swellings (see Fig. 3) corresponding to the 1st, 2nd and 3rd visceral 

 pouches. The last named is the least prominent from this view since 

 it is on a lower level and partly hidden by the projection of the 2nd. 

 At this stage the 4th is well developed but much lower (caudal) and 

 hidden from view by the 3rd. 



Seen from the side, as in Fiff. 2. the model bends almost at right 



