548 



THE EMBRYOLOGY OF THE FROG 



fledged frog with all its various organs and the form that it is to retain 

 throughout adult life. 



To grasp fully that which follows, it is necessary to review the ac- 

 count of the reproductive organs in the chapter on the frog in the early 

 part of this book. 



As the sperm from the male frog never enter the female body, the 

 egg must be fertilized after it has been laid. This is quite different from 

 fertilization in the hen. During the breeding season, as the eggs are 

 squeezed from the female, the male passes over the eggs and deposits 

 his sperm upon them. The fertilized eggs begin to divide almost imme- 

 diately, and within approximately thirty-six hours the blastula stage has 

 been reached. In about six days, when the embryo is five millimeters in 

 length, there is already a twitching within the egg, showing that life is 

 present, and within two weeks after fertilization the embryo wriggles 

 its way out of the surrounding jelly and becomes a free-living larva or 

 tadpole. This is the end of the true embryonic period. 



If the temperature is 

 greater than normal, such as 

 it usually is in the labora- 

 tory, then the larvae may 

 hatch in five days. In either 

 case, however, suckers 

 (Figs. 316, 317, 318) are 

 formed in the head of a tad- 

 pole by which it attaches 

 itself to a jelly-like substance 

 surrounding the eggs al- 

 though it may attach itself 

 to other objects in the water 

 as well. Sometimes the tad- 

 poles will even fall to the 

 bottom of the water and lie there. 



As the mouth opening does not form until two to five days after 

 hatching, the tadpole naturally cannot take in any food from the out- 

 side, and so is still dependent upon the yolk, still undigested, within its 

 body. 



As soon as the 

 tadpole begins to take 

 in food from the out- 

 side, the suckers de- 

 teriorate and disap- 

 pear. From this time 

 on the tadpoles are 

 very active They feed 

 on almost any plant or 



Fig 316. 



A frog embryo at the stage of hatching. an., 

 Proctodseum ; au.c., slight swelling over the rudiment of 

 the ear ; e.g., external gills on gill arches ; na., invagina- 

 tion to form nasal capsule ; o.c., slight swelling over the 

 rudiment of eye; s., sucker; atm., stomodseum (in- 

 vagination which will form the mouth . (After Bor- 

 radaile. ) 



SpT 



Fig. 317. 



Four stages of the development of the adhesive apparatus; 

 (suckers) of Bufo vulgaris; A, suckers; M, mouth; SpT, 

 spiracular tube. In 3 the gills are almost completely hidden by 

 the united right and left opercular folds. 



The small outline figures indicate the shape and approxi- 

 mate size of the tadpoles. (After Thiele.) 



