266 COMPARATIVE ANATOMY 



layer of the latter. At first the two layers of the folds are 

 in close contact with one another. The result of the out- 

 growth of the five pairs of gill-pouches is the formation of 

 four pairs of visceral or gill-arches lying between them. The 

 external wall of each arch is covered by epiblast, its internal, 

 anterior, and posterior walls by hypoblast. An arch is also 

 formed as a thickening in front of the first gill-slit, and 

 a similar but imperfectly defined arch behind the fifth gill-slit, 

 making six arches in all on each side of the throat. The first 

 is the mandibular arch. Subsequently a bar of cartilage is 

 developed in it, which undergoes peculiar changes, and gives 

 rise to the jaws. The second arch is the hyoid, the remainder 

 are the first, second, third, and fourth branchial 'arches, 

 respectively. The first gill outgrowth is the hyomandibular 

 cleft. Lying between the mandibular and hyoid arches, it 

 corresponds to the spiracular cleft of the dogfish, but it never 

 acquires an opening to the exterior in the frog. In the 

 succeeding stages it recedes from the epiblast, acquires a wide 

 lumen, and eventually gives rise to the tympanic cavity and 

 Eustachian passages of the adult. Thus we learn that the 

 last-named structures are the homologues of the spiracle of the 

 dogfish. The remaining outgrowths are the branchial clefts, 

 and they all acquire openings to the exterior. The second and 

 third clefts open first, the first somewhat later, and the fourth 

 last of all. 



At the time of hatching, the first and second pairs 

 of branchial arches bear each a branched and richly 

 ciliated process or external gill, and somewhat later, similar 

 gills are developed on the third branchial arches. After the 

 gill-slits have opened to the exterior, these external gills shrivel 

 up and are replaced by new external gills formed lower down 

 on the branchial arches. 



The lungs are formed from the ventral wall of the en- 

 teron, immediately behind the pharynx. They appear quite 

 early in larval life, at about the time of hatching, but for some 

 time are very small and functionless. In later larval life they 

 extend as a pair of thin-walled sacs into the ccelomic cavity, 

 receive a special blood supply and function as respiratory 

 organs. 



The mesoblast gives rise to the musculature, the coelomic 

 epithelium, the skeleton, and the reproductive, vascular, and 



