THE EARLY DEVELOPMENT OF THE FROG 63 



oxus) and among the Elasmobranchs, Teleosts, and all of the 

 Amniota, are in various respects quite special. Many of these 

 special conditions may be more easily understood and com- 

 pared through common reference to the Amphibia so that as a 

 form transitional between the lower and higher Craniates the 

 frog is a type of great importance. 



There are also practical and historical reasons for empha- 

 sizing the development of the frog. The size of the eggs and 

 their abundance at a season convenient for their study, the 

 hardiness of the embryos under laboratory conditions and 

 during experimental manipulation, and the ease with which 

 the eggs may be fertilized in the laboratory, all make the frog's 

 egg a particularly valuable laboratory subject. For such 

 reasons this egg has served as the basis for many of the great 

 embryological classics; in this respect the egg of the frog is 

 second only to that of the fowl. And much of the important 

 modern experimental embryology has had this same object 

 as its basis, so that a thorough knowledge of the development 

 of the frog is essential to the student of biology. 



I. OUTLINE OF THE LIFE HISTORY OF THE FROG 



It will prove advantageous to recall, at this point, the most 

 striking facts relating to the life history and development of 

 external characters of the frog. The later development of 

 this animal is marked by several abrupt changes in habit, 

 accompanied by pronounced external modifications, but the 

 earlier development is not so obviously divided into periods, 

 marked by striking changes in habit or structure. The whole 

 period of development may be subdivided as follows: 



I. THE FORMATION AND PRODUCTION OF THE GERM CELLS. This 

 period terminates with spawning. 



II. THE EMBRYONIC PERIOD. This is conveniently divided into: 



A. From Spawning through Gastrulation and Notogenesis. 

 This includes fertilization, cleavage, the formation of the 

 germ layers, the formation of the neural tube and noto- 

 chord, and the establishment of an early embryo. 



B. From the Early Embryo to the Time of Hatching. 



