THE RELATIONSHIPS AND HISTOGENESIS OF THY- 

 MUS-LIKE STRUCTURES IN AMMOCOETES^ 



IVAN E. WALLIN 



Department of Anatomij and Biology, Marquette Ufiiversity Medical School 



THREE TEXT FIGURES AND FOUR PLATES 



CONTENTS 



Introduction 127 



Literature 129 



Material and methods 131 



Comparative anatomy of the Ammocoete pharynx 13? 



Organogenesis of the pharynx in the Ammocoetes 137 



Histogenesis 1'13 



Early development of the. pharyngeal wall in Petromyzon marinus uni- 



color, Lampetra wilderi 1'13 



Epipharyngeal placodes 146 



Development of lymphocytes 152 



Histogenetic comparisons between cells arising from placodes and ■ 



lymphocytes 154 



Discussion 155 



Summary 157 



Bibliography 158 



INTRODUCTION 



The question of the source and mode of development of the 

 component structures of the thymus is one of the most difficult 

 problems in anatomy. Although a vast literature has accumu- 

 lated during the last fifty years some of the most fundamental 

 phases of the problem still remain unsolved. 



The thj^mus has been described in every group of vertebrate 

 animals from the elasmobranchs up to and including man. The 

 involvement of gill pouches in the formation of the gland has 

 been established in almost all of the investigated forms. There 



1 This thesis has been accepted by the Graduate School of New York Univer- 

 sity, in partial fulfillment of the requirements for the degree of Doctor of Science. 



127 



