140 



EMBRYOLOGY 



MOUTH 

 OPERCULUM 



Fig. 81. Lateral and ventral views of a stage- 2 3 Rana sylratica tadpole. 

 (Courtesy Professor Arthur W '. Pollister, Columbia University.) 



The larva takes in food, grows for a period of a few months, and then 

 undergoes metamorphosis into a frog. This process is under the control of 

 the thyroid gland and involves a resorption of the tail by the body, the 

 emergence of the forelimbs from the gill chamber, changes in the form of 

 the head, loss of gills, and numerous other form changes. It is interesting 

 that the discovery of the function of the thyroid gland came about by feeding 

 various glands, including the thyroid, to tadpoles. Those eating thyroid 

 metamorphosed long before their normal time, and miniature frogs resulted. 

 The precocious metamorphosis of frog tadpoles may be obtained by adding 

 thyroxin to the water. Doses as small as 1 part thyroxin to 1,000,000 parts 

 water are effective in speeding up metamorphosis. 



In the course of this brief presentation of frog development many new 

 problems arise for analysis. For about the time of hatching the embryo 

 becomes a functioning organism. The muscles contract, the heart beats, and 

 the larva responds to stimuli. How do the structures which have formed 

 according to the principles outlined in the early chapters coordinate to per- 

 form certain functions? With the next chapter we shall begin a study of the 

 problem of the interaction and integration of structural units to form 

 functioning systems. 



