The Pharyngeal Pouches in the Mammalia 237 



He, however, does not trace its transformations in the pig, merely 

 stating that he had found it and its outer connective in a 19 mm. 

 embryo. 



D. THE FOURTH PHARYNGEAL AND ITS TRANSFORMATION INTO THE 

 LATERAL THYROID AND GLANDULE THYROIDIENNE. 



In a 6.2 mm. cat we noticed the division of the fourth pouch into two 

 segments by a lateral constriction (Figs. 58-59). The more dorsal of 

 these constitutes the body of the pouch; it projects strongly backwards 

 and on its outer side gives off a slender process, which connects with the 

 ectoderm of the inner extremity of the sinus preecervicalis (Fig. 60). 

 The remaining segment forms a ventral diverticulum •(V.D.4), which 

 at this period extends for a short distance downwards, forwards and 

 inwards. 



The 9.7 mm. cat shows nearly similar conditions. The ventral diver- 

 ticulum (Fig. 61, V.D.4) has lengthened slightly and begins to assume 

 a more tubular aspect. The dorsal extremity forms a more distinct 

 posterior process. The lateral process still persists, and, as before, 

 connects with the ectoderm of the fundus prsecervicalis. 



In a 10 mm. pig (Student collection. Harvard) the fourth pharyngeal 

 pouch of one side has entirely separated from the ectoderm, while on the 

 opposite side the slender connecting process still persists (Figs. 7-8). 

 The dorsal process (Gl.T.) projects strongly backwards. The ventral 

 diverticulum forms a relatively short, rounded protuberance. Its ventral 

 extremity lies immediately under the aortic arch. It bears the same 

 relation to the latter that the corresponding part of the preceding pouch 

 does to the carotid arch. 



In a second 10 mm. pig (No. 401, Harvard series) the fourth pouch 

 of each side has severed its connection with the ectoderm (Figs. 4-6). 

 The ventral diverticulum (V.D.4) forms a compressed, antero-ventrally 

 projecting sac. The lateral process has disappeared. The dorsal pro- 

 cess (Gl.T.) retains the same characteristics as hitherto. 



A 12 mm. pig is apparently exceptional in that the fourth pouch of 

 one side retains its connection with the fundus prsecervicalis (Fig. 10). 

 On the opposite it has essentially the same features as in the preceding 

 stage. 



In two pigs of 13 and 14 mm., respectively, the fourth pouch has 

 been transferred to a considerably lower level by the downward flexure 

 of the sinus piriformis. The ventral diverticulum (Figs. 14 and 20, 

 La.T.) shows increased length and forms a tubular process, whose axis 



