256 PROCEEDINGS OF THE ACADEMY OF [March, 



3. At first the outer or distal edge of the hyomandibular fold 

 is smooth and uubrokeii throughout its entire extent. Later, this 

 edge becomes interrupted in its middle portion by the formation of 

 a progressively deepening depression, which ultimately reaches the 

 pharyngeal wall and divides the hyomandibular fold into two 

 parts — a dorsal cord-like portion, the future tyrapano-Eustachian 

 passage, and a ventral portion forming a shallow sacculation to 

 the ventro-lateral portion uf the pharyngeal cavity. 



4. The ventral portion of the hyomandibular fold ceases to be 

 recognizable after the late tadj)ole stages. It is this portion which 

 Villy considers as the last remnant of the hyomandibular fold. 



5. The earliest evidence of the degeneration of the hyoman- 

 dibular fold is afforded l)y the recession of its outer edge from the 

 neighborhood of the external ectoderiu. Only the dorsalmost 

 portion of the fold continues in intimate proximity to the skin. 

 The Avithdrawal of the remainder is associated with (1) the reduc- 

 tion in size of the pharynx, in consequence of the segregation of 

 the surrounding mesenchyme to form the Anlagen of muscles and 

 cartilages, and (2) the development of the muscles of the hyoid 

 arch — the depressor mandibuke and ossis hyoidei. Of these mus- 

 cles the depressor maudibulre extends forward between the skin 

 and the outer border of the hyomandibular fold and acquires 

 attachment to the developing quadrate cartilage in front of the 

 fold. It thus interposes an effective barrier to further outward 

 extension of the fold. Only the dorsalmost portion of the fold 

 remains unimpeded by the muscle, and this accordingly retains its 

 proximity to the ectoderm and in the subsequent growth of the 

 head is cari'ied outward as a narrow, cord-like strand expanded at 

 its outer extremity into a club-shaped swelling. This portion I 

 have designated the "diverticulum." It is th.^ Anlage of the 

 tympano -Eustachian passage. 



6. The outer hyoidean muscle, the depressor ossis hyoidei, also 

 acquires attachment to the quadrate Anlage at a point above and 

 posterior to the hyomandibular cleft. The "diverticulum" or 

 Anlage of the tympauo-Eustachian passage thus comes to he 

 between the two hyoidean muscles. 



7. The growth anteriorly of the hyoidean muscles produces a 

 marked antero-posterior extension of the hyomandibular fold and 

 of its derivative, the tyrapano-Eustachian Anlage. This antero- 



