CONTENTS 



Preface page ix 



Acknowledgments xiii 



Chap. I Historical introduction to the problem of differ- 

 entiation I 



II Early amphibian development: a descriptive 



sketch 1 3 



III Early amphibian development: a preliminary ex- 



perimental analysis 35 



IV The origin of polarity, symmetry, and asymmetry 60 

 V Cleavage and differentiation 83 



VI Organisers : inducers of differentiation 134 



VII The mosaic stage of differentiation 194 



VIII Fields and gradients 271 



IX Fields and gradients in normal ontogeny 312 



X Gradient-fields in post-embryonic life 354 



XI The further differentiation of the amphibian 



nervous system 373 



XII The hereditary factors and differentiation 397 



XIII The prefunctional as contrasted with the func- 



tional period of development 418 



XIV Summary 438 



Bibliography and index of authors 443 



Appendix 481 



Index of subjects 499 



