164 OUTLINES OF CHORDATE DEVELOPMENT 



bodies. These remain present throughout life, lying just 

 below the aortic arches (Fig. 56). 



A body described as the pseudothyroid body appears in the 

 postero- ventral branchial region; this seems to have no relation 

 with the remains of the disappearing gill clefts. In addition to 

 the dorsal remains of the gill clefts, mentioned above, mid- 



ep 



FIG. 57. Semi-diagrammatic optical section of the head of a 7.5 mm. larva 

 of R. temporaria, illustrating the relations of the visceral pouches and chondro- 

 cranium. After Spemann. The wall of the pharynx toward the observer has 

 been removed, so that the visceral arches are shown in section, a, Auditory 

 organ; ac, anterior ascending process of the palato-quadrate cartilage; e, eye; 

 E, rudiment of Eustachian tube; ep, epiphysis; h, hypophysis; hy, hyoid cartilage; 

 ra, oral membrane; md, mandibular cartilage; n, notochord; o, olfactory organ; 

 os, oral sucker (in section); p, pharyngeal cavity; pq, palato-quadrate cartilage; 

 s, stomodseum; t, trabecular cartilage; tc, trabecular cornu; th, rudiment of 

 thyroid body; ty, rudiment of thymus; 1-4, first to fourth visceral pouches. 



die and ventral traces may be seen for some time after 

 metamorphosis. 



A pair of ultimobranchial bodies, known also as post-branchial 

 or suprapericardial bodies, are found in the frog, lying posterior 

 to the fifth visceral pouch. These are formed as solid pro- 

 liferations from the pharyngeal wall just back of the fifth 

 visceral pouch (fourth branchial pouch). They clearly repre- 



