THE MICROSCOPIC STRUCTURE OF THE YOLK-SAC 

 OF THE PIG EMBRYO, WITH SPECIAL REFERENCE 

 TO THE ORIGIN OF THE ERYTHROCYTES 



H. E. JORDAN 



Department of Anatomy, University of Virginia 



THIRTY-FIVE FIGURES (tWO PLATES) 



CONTENTS 



I. Introduction 277 



II. Material and methods 278 



III. Descriptive 279 



a . The entoderm 279 



b. The mesothelium 280 



c. The mesenchyma 281 



d. The endothelium 281 



e. The blood cells 283 



1. Terminology 283 



2. Haemoblasts 283 



3. Erythroblasts 286 



4. Normoblasts 287 



5. Giant cells 287 



IV. Discussion 289 



a. The function of the yolk-sac 289 



1. Digestive 289 



2. Haemopoietic 292 



b. The giant cells ' 295 



V . Summary 297 



VI. Literature cited 298 



I. INTRODUCTION 



The chief purpose in view in this study of the yolk-sac of the 

 pig embryo was the acquisition of further data regarding the 

 earhest stages in blood cell origin and development in mammals. 

 The yolk-sac was believed to be the most favorable material 

 for the search for evidence concerning the disputed relationship 

 between mesenchyma, primitive endothelium and haemoblasts. 

 The pig embryo was selected for study on account of its ready 

 a\'ailability. It was hoped that information could be contrib- 



277 



THE AMERICAN JOUKXAL OF AXATOMV, VOL. 19, NO. 2 



