164 THE FROG. 



3. The Thymus. 



The thymus arises in tadpoles of about 8 mm. length, 

 shortly after hatching, as a pair of epithelial buds from the 

 wall of the pharynx, opposite the dorsal ends of the first 

 branchial clefts. Soon after the opening of the mouth, these 

 buds separate from the epithelium as a pair of solid rounded 

 bodies, formed of deeply-staining epithelial cells, which lie 

 imbedded in the roof of the mouth, below the anterior ends of 

 the auditory vesicles, and between the ganglia of the facial and 

 glosso-pharyngeal nerves. 



In each thymus a distinction early appears between an outer 

 cortical layer of small deeply pigmented cells, and a central 

 medullary portion consisting of large pale granular cells. At a 

 later stage the distinction becomes less evident, owing to the 

 cortical cells extending inwards through all parts of the 

 thymus. 



The thymus lies behind the quadrate cartilage, and is 

 carried backwards by the rotation of this cartilage which accom- 

 panies the widening of the mouth at the time of the metamor- 

 phosis. The thymus is larger in the tadpole than in the frog, 

 and undergoes degenerative changes after the metamorphosis. 

 In the frog it lies behind the ear and the tympanic membrane, 

 and slightly ventral to these. 



Buds similar to those from which the thymus is formed are 

 developed opposite the dorsal ends of the hyomandibular clefts, 

 simultaneously with the thymus buds ; and at a slightly later 

 stage opposite the second and third branchial clefts as well. 

 These all disappear before the metamorphosis and take no part 

 in the formation of the adult thymus. 



4. The Post-branchial Bodies. 



A pair of small diverticula of the floor of the pharynx arise, 

 in tadpoles of about 8 mm. length, behind the last gill-clefts, 

 and at the sides of the glottis. These soon separate from 

 the epithelium as a pair of small vesicular bodies, lined by 

 cylindrical epithelium ; they disappear shortly after the metamor- 

 phosis. It is possible that they represent, in a modified form, a 

 fifth pair of branchial clefts. 



