158 IVAN E. WALLIN 



elasmobranchs and are to be considered primitive thymus 

 structures. 



The lymphocyte-hke cells which originate in the primitive thy- 

 mus placodes have different morphological characters and have a 

 different mode of formation than the lymphocytes which are 

 formed in the gill arches and lamellae. 



This investigation has been pursued in the laboratories of anat- 

 omy at Cornell University Medical School and Marquette Uni- 

 versity Medical School. While I hope to have established a prim- 

 itive thymus structure in the ammocoetes, many of the important 

 problems of the histogenesis of the lymphocytes and primitive 

 thymus cells must be left undecided until more exhaustive 

 investigations can be completed. 



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