pits which later become the nostrils. On either side of the head appear 

 swellings which become the external gills. The eyes do not yet appear. 



As development goes on the external gills appear as branched 

 organs, two or three on a side; the eye shows as a ring beneath the 

 skin; and the tail grows and presents a middle muscular portion 

 where the muscle segments clearly show. This middle part supports a 

 thin, wafer-like tail-fin the parts of which are called, respectively, the 

 lower and upper crests. The nasal pit shifts in position and becomes 

 the nostril, and the vent opens. The mouth appears, and dependence 

 on the yolk of the belly ceases. Soon the external gills begin to dis- 

 appear, a lateral flap or fold of skin connects the head with the body, 

 and the neck region disappears. Beneath this fold internal gills de- 

 velop. Usually on the left side, but on the middle line in the belly in 

 ribbed frogs and narrow-mouthed toads, the flap does not close 

 completely, but leaves an opening, the spiracle. The water passes into 

 the mouth over the internal gills and out of this hole. On the mouth a 

 membranous, fringed lip, with upper and lower portions (labia) comes 

 into being. At the portal are horny jaws or mandibles. On the upper 

 and lower portions are ridges of horny teeth. The eyes are no longer 

 covered pigmented rings, but are now at the surface. The intestine 

 has become much elongated and coiled, and in some can be seen 

 through the skin. The skin of the back and head comes to have a 

 series of sense organs (or lateral line dots). 



The buds of the hind limbs begin to appear. The fore limbs start 

 to develop beneath the skin. When the hind limbs have reached con- 

 siderable size, the left arm comes out through the spiracle, or the 

 skin breaks down and later the right arm breaks through the skin 

 or the skin weakens for its egress. Normally it is held that the left 

 arm comes out first. Often the right arm appears first. 



The process of transformation is now on. The tail crests decrease 

 in size and the creature begins to live on its tail — that is, to absorb it. 

 The gills vanish, and the lungs begin to serve as the sole respiratory 

 organs, if the skin be not considered. The tadpole appears more and 

 more at the surface or near the shore. The eye assumes eyelids. The 

 tadpole mouth fringe, with its horny jaws and horny teeth, is dis- 

 carded, and a true frog mouth begins to appear. The long intestine 

 becomes wonderfully shortened, for a carnivorous diet, and the small 

 frog, with a vestige of a tail, is ready to leave the water. This process 

 is termed transformation or metamorphosis. 



Notes. These are customarily from our field notes. They treat of 

 habitats, general habits, or breeding, sometimes of experiences in 

 collecting the frogs, or of their enemies or their usefulness and as- 

 sociated ecological features. Occasionally, you will find comparisons 

 with closely related forms. In a difficult group such as that of the 

 swamp cricket frogs (Pseudacris) a description from life, using Ridg- 

 way's color code, is given under each species. 



12 



