AN INTRODUCTION TO 

 VERTEBRATE EMBRYOLOGY 



CHAPTER I 



THE DEVELOPMENT OF THE FROG 

 INTRODUCTION 



THE eggs of the common frog (Rana vir- 

 escens and allied species) may usually 

 be found without difficulty in small 

 ponds and pools of water, where they are laid, in 

 the early spring, soon after the melting of the 

 ice. They may easily be kept alive, in the 

 laboratory, in shallow dishes of water, and their 

 entire development thus observed. Their rate 

 of development varies greatly with the tem- 

 perature, so that if it be desirable, for any 

 reason, to hurry the development, all that is 

 necessary is to place the aquarium in a warm 

 place ; or if it be desired to keep a lot of eggs 



