46 MORPHOLOGY OF THE VERTEBRATE BODY 



both are openings from the back of the mouth cavity. When, 

 however, the development of the mouth region in a frog or other 

 air-breathing vertebrate is examined, an explanation of this seem- 

 ing diversity of structure in frogs and fishes is discovered. In the 

 early stages of its development, the frog possesses gills and gill 

 slits exactly comparable with those of a fish (Fig. 25). When 

 the tadpole changes into the miniature frog, the gills disappear, 

 and the gill slits, with a single exception, become closed, while the 

 bony gill bars degenerate or become incorporated into portions of 

 the skeleton in this region. The most anterior slit, however, 

 remains as an opening extending laterally from the back of the 

 mouth, and later becomes closed externally by a membrane, the 



Fig. 26. — Lung fish, Protopterus anncdens. About one-half natural size. 



(After Dean, from Parker and Haswell, "Textbook of Zoology," copyright, 1921, by 

 Macmillan and Co., Ltd., reprinted by permission.) 



drum of the ear. Thus, in the development of every individual 

 frog, the Eustachian tubes arise by the " making over " of a pair 

 of the gill slits possessed by the embryo. The most anterior pair 

 of gill slits of a fish and the Eustachian tubes of a frog are, there- 

 fore, homologous, that is, they are structurally smiilar. A Uke 

 mode of development occurs in all the higher vertebrates. In the 

 human embryo there is no swimming stage like the tadpole; but 

 early in the development (Fig. 287, p. 531) there are vestigial gill 

 slits, which later disappear or are made over into other structures, 

 notably the Eustachian tubes. The latter are survivals of the 

 anterior pair of gill slits in man as in the frog. 



The mouth and related structures are thus seen to be built 

 on the same anatomical plan in all vertebrate animals. The dif- 

 ferences are explained when one understands the modifications in 

 development, whereby parts have been lost or so changed that 



