CHAPTER II 

 THE EARLY DEVELOPMENT OF THE FROG 



PAGB 

 INTRODUCTION . . . . 62 



I. OUTLINE OF THE LIFE HISTORY OF THE FROG .... 63 



II. THE GERM CELLS AND THEIR PRODUCTION 68 



A. THE EGG AND SPERM AT THE TIME OF SPAWNING . . 68 



B. THE FORMATION OF THE GERM CELLS 71 



1. The Reproductive Organs 71 



2. Oogenesis 73 



3. Spermatogenesis 77 



C. SPAWNING 77 



III. THE EMBRYONIC PERIOD 79 



A. FROM FERTILIZATION THROUGH GASTRULATION .... 79 



1. Fertilization and the Development of the Symmetry of 



the Egg 79 



2. The Symmetry of the Egg 83 



3. Cleavage 91 



4. The Blastula 94 



5. Gastrulation and Notogenesis 98 



6. The Mesoderm 107 



7. The Medullary Plate 110 



8. Summary and Comparisons with other Forms Ill 



B. THE FORMATION OF THE EARLY EMBRYO 116 



1. The Nervous System 116 



2. The Notochord 121 



3. The Enteron 122 



4. The Mesoderm . 123 



The frog (Rana, sp.) is important zoologically, not as a 

 central type representative of any large group of Chordates, 

 but as a transitional form connecting the lower and higher 

 groups of Craniata. This relation is no less apparent embryo- 

 logically than morphologically. For there are comparatively 

 few groups Lampreys, Ganoids, Dipnoans whose develop- 

 ment can be compared closely with that of the Amphibia, while 

 the types of development seen in the Cephalochorda (Amphi- 



62 



