CHAP. V THE DEVELOPMENT OF THE FROG 109 



a pouch-like diverticulum were brought into contact. At 

 their outer ends the slits come into contact with the ecto- 

 derm, with which they fuse. The first two slits are the first 

 to form ; the others appear in order from before backward. 

 When the tadpole escapes from the jelly into the water, the 

 walls of the solid gill slits separate, the ectoderm breaks 

 through at the outer end, and a free communication is estab- 

 lished between the throat and the outside. The first slit, 

 the hyomandibular, does' not break through to the outside; 

 its entodermic lamellae separate and form a pouch which 

 communicates with the pharynx. In most forms the hyo- 

 mandibular cleft forms the Eustachian tube and its covering 

 over its outer end, the tympanic membrane ; but in the frog, 

 according to Marshall, the Eustachian tube has a different 

 method of origin. (See "Vertebrate Embryology," p. 143.) 

 The four following slits are known as the branchial clefts ; 

 of these the second and third open first, then the first, and 

 finally the fourth. 



The thyroid gland begins as a longitudinal groove along 

 the floor of the pharynx. It gradually sinks below the sur- 

 face and becomes converted into a solid, elongated mass of 

 cells. Later it divides into right and left portions, which 

 are completely separated. 



The thymus, according to Maurer, arises by a sort of 

 budding process from the epithelium of the dorsal end of 

 the first branchial cleft. The end then separates from its 

 point of origin and becomes carried backward, finally lying 

 behind the tympanic membrane. The thymus is relatively 

 larger in young frogs than in older ones. Other bodies of 

 similar epithelial origin from the gill clefts are, according to 

 Maurer, the post-branchial bodies, the epithelial bodies, and 

 the pseudothyroid ("ventraler Kiemenrest "). 



The entoderm forms only the inner portion of the alimen- 



