THE DEVELOPMENT OF THE THYMUS IN THE PIG 



11. HISTOGENESIS 



J. A. BADERTSCHER 



From the Department of Histology and Embryology, Cornell University, 



Ithaca, N. Y. 



THREE PLATES (EIGHT FIGURES) 



CONTENTS 



Introduction 437 



Material and methods 438 



Historical 440 



Histogenesis 443 



1. The purely epithelial epoch 444 



2. The epoch of lymphocyte infiltration and lymphocyte proliferation and 



the formation of the reticulum 448 



3. The epoch of the formation of red blood-cells and granular leucocytes. . 465 



Conclusions 484 



Bibliography 486 



INTRODUCTION 



There is perhaps no organ m the body whose mode of devel- 

 opment has given rise to so bitterly contested and so widel}' 

 divergent views as has that of the thymus. This is particularly 

 true of its histogenesis. The source and nature of the small 

 round cells that make up the greatest mass of the organ in its 

 fully developed condition; the origin and nature of its intra- 

 lobular supporting structure; the origin and significance of its 

 granular cells and of the Hassall's corpuscles; the extent to which 

 red blood-cells and granular leucocytes are formed in it; have 

 during the past thirty-five years attracted the attention of many 

 investigators, and yet the only point upon which they unanimously 

 agree is that the thjmius is, in part at least, of epithelial origin. 

 This disparity of views cannot be due to differences in the devel- 

 opment of the thymus in different animal forms, for some workers 



437 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 17, NO. i 



