198 



THE ENDODERMAL DERIVATIVES 



Pharynx Dorsal aorta 



Anterior 



Cranial n 



Otic copsu 



Aortic arch 



Remnant of 

 external gills 



Opercular 

 groove 



Opercular char 



Internal gills Heart 



Pericordial cavity 



Relation of the pharynx to the internal and external gills of the frog, transverse 



section. 



Spiracle (left 

 side only) 



of the roof. This provides a lengthwise pair of pockets between the 

 pharynx and the latero-ventral gill chambers. From the floor of each 

 of these pockets develop finger-like, endodermally covered, frilled 

 organs known as the gill rakers. These tend to filter the water as it 

 passes from pharynx to gill chamber, removing any relatively large 

 particles of matter and retaining them within the pharynx to be carried 

 into the oesophagus with the food. In addition, the shelf-like projec- 

 tion or flap of tissue from the floor of the pharynx and also from the 

 lateral wall of the pharynx likewise aid in the mechanical sifting or 

 filtering of the water. These shelves are known as the velar plates. 

 During subsequent metamorphosis both the gill chamber and the 

 related opercular chamber become filled with rapidly multiplying cells 

 which ultimately become incorporated into the body wall. 



The Thymus Gland. 



The glandular derivatives of the various branchial pouches may 

 be classified as epithelioid bodies, since they are all lined with endo- 

 dermal epithelium. The most anterior derivatives are the paired thymus 

 glands which arise by the proliferation of cells from the dorsal ends 

 of the hyomandibular and the first branchial (second visceral) 

 pouches. The bulk of the gland of the adult comes from the cells of 

 the first branchial pouch, rather than from the hyomandibular (first 

 visceral) pouch, as a solid internal proliferation of cells from the upper 

 lining. At about the 12 mm. stage the sac-like outgrowths separate 



