liJOl.] NATURAL SCIENCES OF PHILADELPHIA. 235 



just anterior to the dorsal origin of the second visceral-cleft (figs. 

 14, 17, 18, Hijm.) and terminating at the position of the future 

 quadrato- mandibular articulation (fig. 10, Hym.). The outer or 

 distal border begins dorsally in continuity with the roof of the 

 pharynx (fig. 14, Hym.), and then extends in a gentle curve 

 downward, outward and forward until it reaches the point where 

 the distal, cord-like extension is given off and which I shall now 

 designate as the " diverticulum." The latter is at first a flattened 

 cord (fig. 11, Eu.), which at first extends forward a short dis- 

 tance, but, when it reaches the anterior border ot the depressor 

 ossis hyoidei, turns sharpl}'^ outward and slightly dowuwai'd in 

 front of the latter and then expands to form a solid, bulbous 

 swelling, which terminates blindly in the mesenchyme a short dis- 

 tance below the external ectoderm (figs. 10, 17, Tym.; also fig. 

 18, Hym.). This portion of the hyoraandibular fold is the only 

 part which comes into close proximity with the external epithelium. 

 The remainder of the fold lies at a considerably deeper level. The 

 distal border of the latter, below the origin of the " diverticu- 

 lum," bends downward and inward and at its ventral end blends 

 with the floor of the jDharyux (figs. 10, 11, Hym.; 12, 13, 14, 

 Hym. [leftside]; 16, 17 [right]). This portion of the distal 

 border is continuous with the ventral border of the " diverticu- 

 lum," and, owing to the slightly downward direction taken by the 

 latter, forms with it a shallow sinus or depression, the concavity 

 of which faces downward and outward. By its anterior surface the 

 hyomandibular fold is in close contact with the quadrate, although 

 partly separated from it by the mandibular aortic arch Qm.a.). 

 Owing to the obliquely anterior direction taken by the hyoman- 

 dibular fold, this surface faces both forward and upward, so that 

 in transverse sections it appears as the dorsal border. Hence it 

 follows that all structures found above the fold are anterior to it, 

 whereas those ventral to it are posterior. The posterior surface 

 faces backward and downward and has in close relation the Anlagen 

 of the hyoid cartilage and associated muscles. Between the two 

 muscles is the ramus hyomaudibularis of the facial nerve {vii h.) 

 which occupies its definitive position posterior to the hyomandibu- 

 lar fold.' 



'The reader will do well to consult figure 3 of Dr. Spemann's paper, 

 •which shows a reconstructiou of the hyomandibular fold of liana teni'poraria 

 at a similar s'age. I find tbat the fold in B'lfo lentiyinosus is in all essen- 

 tial respects similar. 



