x CONTENTS 



A mop of the early gastrula, 59 



Rearrangement of the presumptive structures during gastrulation, 62 



Gastrulation and the formation of the nervous system, 66 



6 • GASTRULATION IN RELATION TO THE ORGANIZER 69 



The anatomy of an amphibian embryo, 70 



The potencies of the ectoderm of an early gastrula, 72 



The induction of the primitive nervous system, 76 



Comparison of amphibian gastrula and chick primitive streak, 80 



7 • THE NEURULA AND THE PROPERTY OF SELF-DIFFERENTIATION 83 



Chemical nature of the organizer, 85 

 Reproduction of primitive nerve cells, 87 

 Head organizer and trunk organizer, 89 

 Theories of organization, 90 

 Self -differentiation of the parts of the neurula, 91 



8 • THE EMBRYO: DIFFERENTIATION WITHIN FIELDS 97 



The polarity of the limb disk, 97 



Modification of polarity, 99 



Labile organization within the limb disk, 99 



Fields and their properties, 101 



Progressive differentiation within a field, 1 04 



9 • DEVELOPMENT OF THE FROG EGG 111 



Breeding habits and fertilization, 1 1 2 



Early cleavages, 1 13 



Late cleavage through gastrulation, 1 16 



Rearrangements of cells during gastrulation, 120 



From early neurula to tail-bud embryo, 1 25 



Segregation of the notochord and mesoderm from the endoderm, 126 



Formation of the neural tube, 129 



The embryo, 1 29 



Structure of the embryo at hatching, 132 



The larva and metamorphosis, 139 



10 • INTERRELATIONSHIPS OF DEVELOPING STRUCTURES 141 



Origin and development of the neuroblasts, 142 



Regeneration of nerve fibers, 146 



Effect of the limb on growth of nerve fibers, 147 



Theories of control of nerve outgrowths, 149 



Effect of the limb on differentiation of neuroblasts, 1 53 



Differentiation within the spinal cord, 1 54 



Common innervation of two limbs, 1 56 



Relation of differentiation to function, 1 57 



