244 PROCEEDINGS OF THE ACADEMY OF [March, 



increased in size and extended its area of attachment to the quad- 

 rate. It has also given off an accessory slip, which extends 

 upward external to the cord-like "diverticulum" to attach to 

 the processus muscularis (fig. 15, m.cl.m'.). Moreover, immedi- 

 ately behind the posterior edge of the depressor mandibulie, the 

 hyoid cartilage is drawn up to form an articulation with the quad- 

 rate, and following this the enlarged mandibular aortic arch turns 

 inward to join with the carotid (fig. 19, 7n.a.) just in front of the 

 point where the "diverticulum" joins the extension from the 

 pharyngeal wall (fig. 19, JEu., left side). Thus changes in three 

 structures have been instrumental in producing the deepening of 

 the depi'ession, i.e., (1) the increase in size and area of attach- 

 ment of the depressor mandibuhe; (2) the articulation of the 

 hyoid to the quadrate, and (3) the increase in size of the man- 

 dibular aortic arch. 



The reduction in size of the middle portion of the " diverticu- 

 lar" cord (compare figures vilth. fig. 11 of last stage) has been 

 associated with two factors: (1) the increase in size of the accom- 

 panying mandibular aorta, and (2) the differentiation and growth 

 of the outer, accessory slip of the depressor mandibulas. By 

 examining figure 15, one will notice the reduced cord tighlly 

 wedged in between the enlarged artery internally and the accessory 

 slip externally. 



The decrease of the ventro-anterior portion of the hyomandibular 

 fold to form a mere shallow protrusion of the ventro-lateral angle 

 of the pharynx (fig. 24, Hym.) has in all probability been pro- 

 duced by the deepening of the depression and its final blending 

 with the pharyngeal wall. Naturally, as the depression deepeued 

 its deepest part would ultimately blend with the pharyngeal wall, 

 so as to be no longer distinguishable (fig. 15). As the ventro- 

 anterior portion of the fold formed the lower border of the depres- 

 sion, it would naturally be drawn in with the deepening of the 

 depression until it formed the shallow protrusion mentioned (fig. 

 24, Hym. ) . This decrease is also accelerated by the increase in 

 size and density of the skeletal and muscular parts. 



Stage V. — Young tadpole of about 9 mm. Opercular cavity 

 communicating with the exterior by a single opening on the left 

 side. No external gills. 



The condition of the hyomandibular fold is essentially similar to 



