TABLE OF CONTENTS. 



PAGE. 



Introduction 315 



Material and Methods 316 



Descriptive 319 



11-day embryo; Flemming fixation, iron-hematoxylin stain 319 



10-day embryo; Helly fixation, Giemsa stain 322 



12-day embryo; Helly fixation, iron-hematoxylin stain 322 



Older embryos; from 13 to 32 days 323 



Younger embryos; from 9 to 6 days 325 



Youngest embryos; from 2 to 5 days 326 



Literature 327 



Germ-cell origin in invertebrates 327 



Germ-cell origin in vertebrates 328 



Discussion and Conclusions 333 



Summary of Results 340 



Bibliography 342 



Description of Illustrations 344 



314 



