THYMUS-LIKE STRUCTURES IN AMMOCOETES 155 



occur in the same section and thus cannot be attributed to differ- 

 ence of technique. A lymphocyte with red cytoplasm may occa- 

 sionally be found in the epipharyngeal ridge just as a 'lympho- 

 cyte' wdth gray-blue cytoplasm may occasionally be found in the 

 gill region. The great majority of 'lymphocytes' in the placode 

 region, however, contain cytoplasm which stains gray-blue. 

 It was also pointed out above that the lymphocytes in the 

 gill region are chiefly the type which have red cytoplasm. The 

 presence of the lymphocytes with red cytoplasm in the placode 

 region and the type with gray-blue cytoplasm in the gill region 

 may be accounted for by migration from their seats of origin. 

 They may also be brought to these situations by the flow of the 

 blood. 



On account of the morphological difference of the develop- 

 ing lymphocytes in the gill region and lymphocyte-like cells of 

 the placodes, the conclusion seems justifiable that the placodes 

 are segregated portions of the 'epithelium' representing indi- 

 vidual organs which produce cells of a lymphocyte appearance, 

 but differing from the lymphocytes formed in the 'epithelium' 

 of the gills. 



DISCUSSION 



The data submitted in the consideration of the lymphocyte 

 accumulations in the lateral branchial wall of the lamprey larva 

 does not supply any evidence that these formations represent 

 primitive thymus anlagen. Although placode-like formations 

 are present in the lateral branchial wall, similar formations are 

 also present in other parts of the pharyngeal epithelium. 



An important component of the thymus of higher animals is a 

 reticulum. In my study of the thymus-like placodes in the lam- 

 prey larva, I have been unable to find any undisputable evidence 

 of a reticulum in the placode. At the connective tissue border 

 of the placode the epithelial cytoplasm apparently has a fibrous 

 character (fig. 10). I have been unable to determine whether 

 this represents connective tissue or transformed epithelial cyto- 

 plasm. Judging by its appearance and position it probably 

 represents connective tissue which has been invaded by cyto- 



