38 F. W. THYNG 



tral ridge, representing the entodernial part of the first closing- 

 plate, separates it from the caudo-ventral surface. Upon the 

 caudo-ventral surface near the tip of the pouch there is a slight 

 groove. This, the 'tensor groove' of Hammar ('02), marks the 

 place of formation of the tensor tendon. 



The second pharyngeal pouch (Ph.P.2, plate 2) appears on 

 either side as a low evagination from the lateral pharyngeal wall. 

 It is situated just caudal to the first pouch and projects towards 

 the cephalic aspect of the glossopharyngeal nerve. According 

 to Hammar ('03) this evagination represents only the dorsal part 

 of the primary pouch. A deeply staining cyst is present in this 

 embryo on either side of the pharynx, w^hich evidently belongs 

 to either the ectodermal or entodernial part of the second bran- 

 chial groove. The left cyst is situated just lateral to the left 

 glossopharyngeal nerve, while the right is just cephalad of the right 

 glossopharyngeal nerve and in close relation to the second pouch. 

 Piersol ('88) found that in rabbit embryos there were formed, in 

 the development of the second pharyngeal pouches, two epithelial 

 tubes on either side, one from the entoderm and the other from 

 the ectodenn, both of which subsequently atrophied. The 

 former, however, persisted longer than the latter. Hammar 

 ('03) described and figured a structure in human embryos pro- 

 truding above the margin of the tonsilar pouch. In early stages 

 this was connected with the ectoderm, and hence he concluded 

 that it was of ectodermal origin. Fox ('08) did not find any 

 ectodermal remnant in this region of the pig embryo, but de- 

 scribed and figured a long filiform process continuous with each 

 of the second pharjmgeal pouches. 



The third pharyngeal pouches (plates 2 and 6) have lost their 

 connection with the pharynx. They are now represented each 

 by a compact cylinder (Thy., plates 2 and 6) in the side of the 

 neck, and which contains only a slight lumen. The cylinders 

 converge caudally toward the median line and end approxhnately 

 at the level of the aortic arch (Arc.ao.). The right and left cylin- 

 ders become, eventually, the corresponding lobes of the thymus 

 gland with the exception of the cephalic extremities which are 



