260 PROCEEDINGS OF THE ACADEMY OP [Maicll, 



Fig. 11. — Fourth section posterior to that of Cgiire 10. Eu. designates 

 the •'diverticulum," while at Ihjm. is the autero-inferior ijortion of the 

 hyoinandibular fold. 



Fig. 12. — Third section posterior to the last. The diverticulum {Eu.) i& 

 now continuous with the antero-iuferior portion (Jli/m.) of the hyomandi- 

 bular fold. Between the two is the depression lodging tlie muscles {m.d.m. 

 and m.dJt.). The sn)all vessel above Eu. is the mandibular aortic arch. 



Fig. 13. — Sixth section posterior to the last. The mandibular aortic arch 

 is dorsal \o llym. 



Plate VIII, Fig. 14. — Third section posterior to last. The mandibular 

 aortic arch on the right side is just internal to vii pi. 



Fig. 15. — Transverse section of head of tadpole of stage IV in the region 

 immediately posterior to that shown in figure 24. 



Fig. 16. — Coronal section of the head of a young tadpole of stage II. On 

 the right side the section passes a slight distance above the floor of the 

 pharynx, Avhiie on the left it is consideraldy higher. The small vessel in 

 front oi Ilym. is the mandibular aortic arch. 



Fig. 17. — Fourth section dorsal to the last. On the right side the little 

 protrusion of the pharyngeal wall just internal to Tym. is the antero-inferior 

 portion of the hyomandibular fold jiist below the point where it becomes 

 continuous with l^ym. The space between the two is the depression. 



Fig. 18. — Coronal section of the head of the same animal a slight distance 

 below the roof of the mouth. 



Fig. 19. — Transverse section of the head of a tadpole of st£tge IV in the 

 region of the processus ascendens. 



Plate IX, Fig. 23. — Transverse section of the head of a tadpole of stage 

 IV, passing through a region slightly anterior to the auditory sac. 



Fig. 24. — Transverse section of the head of a tadpole of stage IV, showing 

 the anterior expanded portion {Tym.) of the Eustachian cord. Hym. de- 

 notes the proximal anterior portion of the hyomandibular cleft. 



I'ig. 25. — Transverse section of the head of a tadpole of stage V. On the 

 left the section is immediately posterior to the eye. The minute upgrowth 

 from the dorsal wall of l.r. just internal to vii h. is the Eustachian 

 proliferation. 



Fig. 26. — Transverse section of a portion of the right side of the head of 

 an old tadpole of 18 mm. (stage VI). This section is considerably more 

 magnified than the others and is intended to show the extremely rudimentary 

 character of the Eustachian cord at this stage. 



Fig. 28. — Transverse section of the right side of the head of a tadpole at 

 the beginning of the metamorphosis (stage VIII), showing the formation of 

 the annular cartilage. 



Fig. 29. — Transverse section of a portion of the right side of the head of 

 the same animal. The section was drawn with the same degree of magni- 

 fication as figure 26 and is intended to show the Eustachian cord when it 

 begins to regenerate. 



Fig. 30. — Transverse section through one side of the head of a young toad 

 (stage XV). The section passes through the region immediately in front of 

 the car-capsule. 



Fig. 31. — Transverse section through one side of the head of the same 

 animal. The section passes through the anterior portion of the ear-capsule. 



