THE LATER DEVELOPMENT OF THE FROG 127 



VII. THE SKELETON AND TEETH 207 



1. The Vertebral Column 208 



2. The Skull 210 



A. The Cranium and Sense Capsules 210 



B. The Visceral Arches 216 



C. The Dermal Elements 219 



3. The Teeth 220 



4. The Appendicular Skeleton 221 



IN this chapter we shall trace the chief events in the develop- 

 ment of the frog tadpole, from the stage described at the close 

 of the preceding chapter, i.e., just after elongation is inaugu- 

 rated by the enlargement of the head and the outgrowth of the 

 tail. The more important changes in external form and in 

 habit have been described in the introduction to the preceding 

 chapter, and we may turn at once to the description of the inter- 

 nal processes of development. 



The developmental history of no single species of frog is 

 known with even fair completeness, and we should note that 

 this chapter presents a composite account of the development 

 of the genus Rana. No attempt has been made, save in occa- 

 sional instances, to distinguish the species serving as the 'basis 

 for different sections; these vary somewhat in details, but since 

 details are largely omitted, little confusion is likely to follow 

 such treatment. The species of Rana chiefly serving as the 

 material for this account are the American species, sylvatica, 

 palustris, and virescens, and the European temporaria, esculenta, 

 fusca-, and muta. 



While the early history of the frog embryo is fairly well 

 known, there are still many gaps in our knowledge of the later 

 development of the tadpole, gaps w r hich sometimes seem of 

 remarkable proportions in view of the extent to which the frog 

 is used as an object of embryological study. 



No accurate and convenient description of the age of the 

 frog larva has been determined, since the rate of development 

 varies so markedly with temperature changes before the opening 

 of the mouth, and afterward with the abundance of food. At 

 the time of hatching, which is usually between one and two 

 weeks after fertilization, the larvae of most species are approxi- 



