256 



GENERAL ZOOLOGY 



about the end of the first month. Both pairs of legs are formed 

 from the body wall, but the forelegs develop within the branchial 

 chamber which makes them invisible externally until metamor- 

 phosis sets in. The hindlegs develop from the lateral body wall 

 at the base of the tail, and their development can be readily 

 followed from the beginning. Actually the forelegs start to 

 develop before the hindlegs. 



Olfactory Organs. — The olfactory passages develop from a 

 pair of ectodermal pits on either side of the head just above the 

 oral invagination of the early larva (Fig. 156). These pits 

 deepen and eventually connect with the pharynx to form the 



internal nares. The olfactory 

 nerve develops from cells lining 

 the olfactory passages. These 

 cells are the olfactory cells, from 

 which axons extend back to the 

 forebrain to form the olfactory 

 nerve. The passages them- 



Fig. 159. — Cross sections illustrating , , , „ . 



two stages in the early development of selves are enlarged to form the 



the inner ear of Rana sylvatica. a.c, nasal Cavities. 



alimentary canal; o.p., otic pit; o.s., ,-, ,-,-,, . , , 



otic sac, developed from the pit and Ear.— the inner ear develops 



which in later stages forms the mem- at about the same time as an 

 branous labyrinth. (After Pollister and . ■ ■, • , i • i ,. 



Moore.) °^ lc P l * m the inner ectoderm ot 



the skin at the level of the 

 hindbrain (Fig. 159). The pit becomes cut off from the surface 

 and later differentiates into the membranous labyrinth. The 

 middle ear develops later through an enlargement of the vestigial 

 first (hyomandibular) gill pouch. This gill pouch, which in 

 fishes is provided with a cleft opening to the outside and serves 

 as a respiratory passage, in the frog never breaks through to the 

 outside. It may therefore be regarded as an incomplete gill 

 cleft. In the frog as the gill pouch enlarges, it encroaches on the 

 surface until its outer wall is reduced to a thin membrane, the 

 tympanic membrane. 



The enlargement forming the tympanic cavity is connected 

 with the gill pouch by a narrow channel, which is the Eustachian 

 tube of the adult frog. The columella of the middle ear is derived 

 from the dorsal wall of the tympanic cavity from which it 

 becomes separated by excavation to extend freely from the 



