A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 1 3f> 



DIGESTIVE SYSTEM. 

 MOUTH. 



The mouth has been partly described in connection with the integument. 

 It is directly continuous with the pharynx, the line of former separation being 

 represented above by a few scattered clusters of cells, and below by a thin ridge 

 of epithelium, remnants of the oral membrane. The oral cavity is broad trans- 

 versely but narrow dorso-ventrally; there is no indication of the anterior lobe of 

 the hypophysis. The scattered cells which Janosik 20 has designated as the hypo- 

 physis are undoubtedly remnants of the oral membrane. 



I shall here mention His's 18 description of his embryo "Lg" (2.15 mm.), in 

 which he recognizes both Rathke's and Seessel's pockets, while the oral membrane 

 is still intact. Concerning these he says: 



"Of the two peaked recesses between which it (oral membrane) passes, the anterior 

 becomes Rathke's pocket, while posterior becomes Seessel's pouch." 



In an embryo of 2.6 mm. in Keibel and Elze's Normentafel 21 , in which the 

 pharyngeal membrane is still present, the hypophysis is "just indicated." The 

 Robert Meyer embryo of 23 segments, according to Thompson, shows no hypo- 

 physis, although the Normentafel states its beginning is "doubtful." 



FOREGUT. 



PHARYNX. 



The cavity of the pharynx is broader and deeper than that of the mouth. 

 In the median plane its dorsal wall lies ventral to the notochord and follows closely 

 the curvatures of that structure, being fused with it posteriorly (text-fig. 7). 

 Ventrally the floor of the pharynx is more irregular. It possesses, toward its 

 anterior end, a short, rounded diverticulum, the beginning of the thyroid gland 

 (plate 2, figs. 2, 3, 4). This is in close relation ventrally to the ventral aorta, with 

 which it lies in contact. Thompson describes a thyroid gland which is appar- 

 ently in a similar stage of development, but Janosik and His (in his embryo Lg), 

 fail to show this organ. 



Behind the thyroid diverticulum the ventral wall of the pharynx shows two 

 shallow depressions which cross the midline (plate 2, fig. 4). These are the cut 

 sections of the transverse grooves (the "ventral pharyngeal grooves" of Grosser) 

 which extend from side to side and connect the pouches of one side with those 

 of the other. As described by Grosser 15 , I find that the ventral groove of the 

 first pouch before reaching the midline divides into two limbs which surround a 

 median elevation. This he identifies as the "tuberculum impar." It is to be 

 noted that the thyro-glossal duct proceeds from the summit of this elevation. 

 This relation is noted by Grosser, who describes it as follows: 



"The opening of the thyroglossal duct is situated at first upon the summit of the 

 tubercle, but later it becomes shifted into the furrow bounding the tubercle posteriorly 

 or, according to Ingalls, in an embryo of 4.9 mm., into 'the region of the second arch, 

 immediately aboral to the tuberculum impar.' ' 



