INTRODUCTION TO THE EMBRYOLOGY OF THE LEOPARD FROG 25 



heat, available oxygen, and pressure. The tadpole soon begins vora- 

 cious feeding on a vegetarian diet. 



Shortly after hatching, finger-like external gills develop rapidly on 

 the posterior sides of the head and these constitute the only respira- 

 tory organs. Simultaneously with the opening of the mouth a series of 

 visceral clefts (gill slits) develop as perforations in the pharyngeal 

 wall, and their walls become folded to form internal gills. The ex- 

 ternal gills gradually lose their function in favor of the internal gills. 

 They then become covered over by a posterior growth of tissue known 

 as the operculum. There remains but a single excurrent pore, the 

 spiracle, on the left side at the posterior margin of the operculum. 

 There are but few changes in the respiratory system from this stage 

 until metamorphosis begins at about IVi months. The internal gills 

 lose their function in favor of lungs at metamorphosis and this allows 

 the aquatic tadpole to become a terrestrial frog. When the tadpole be- 

 gins to develop its lungs it frequently comes to the surface for air. 

 The forelimbs begin to grow through the operculum, and, about 2^/^ 

 months after the eggs are fertilized, the hind legs begin to emerge and 

 the tadpole is ready for the critical respiratory and excretory changes 

 that accompany metamorphosis. 



Metamorphosis in the leopard frog, Rana pipiens, occurs in from 

 75 to 90 days after the egg is fertilized, generally in the early fall and 

 at a time when the food becomes scarce and the cool weather is im- 

 pending. Metamorphosis is one of the most critical stages in frog de- 

 velopment, involving drastic changes in structure and in function of 

 the various parts of the body. The tadpole ceases to feed; loses its outer 

 skin, horny jaws, and frilled lips; the mouth changes from a small oval 

 suctorial organ to a wide slit and is provided with an enlarged tongue; 

 the eyes become enlarged; the forelimbs emerge; the abdomen shrinks; 

 the intestine shortens and changes histologically while the stomach and 

 liver enlarge; the diet changes from an herbivorous to a carnivorous 

 one; the lungs become the major respiratory organs with the moist 

 skin aiding; the mesonephros assumes greater function; the tail re- 

 gresses; sex differentiation begins; and the tadpole crawls out of the 

 water as a frog. 



We may now summarize the steps in the development of the frog as 

 follows: 



1 . Fertilization of the egg 



2. Formation of the gray crescent due to pigment migration 



