THYMUS-LIKE STRUCTURES IN AMMOCOETES 157 



such an interpretation and giving us, I believe, a better comprehension 

 of its morphologic significance, we have the fundamental plan of its 

 histogenesis. 



The true natui'e of the endodermal-mesenchymal relationship 

 in the ammocoete pharynx has not been definitely determined. 

 Whatever these extrinsic factors may be, they are apparently of 

 the same nature in the thymus-like placodes and the lympho- 

 cyte-forming 'epithelium' of the branchial arches. The prod- 

 ucts of these two regions, however, are not similar and it seems 

 to me that this dissimilarity can only be explained on the basis 

 of an intrinsic value or specificity of the 'epithelium' of the 

 placode. 



The nature of the formation of lymphocytes and the primitive 

 thymus placodes in the lamprey larva point to an ontogenetic 

 relationship in the histogenesis of thymus cells and lympho- 

 cytes. The branchial region of the lamprey larva may be looked 

 upon as possessing general lymphocyte-forming properties in 

 which the primitive thymus placodes represent specialized regions 

 of the general lymphocyte-forming 'epithelium.' 



SUMMARY 



From the evidence obtained in this investigation of the 

 ammocoetes the following conclusions seem justified: 



The placodes in the lateral branchial wall are apparently 

 patches of degenerating epithelium and have nothing to do with 

 a thymus structure. The collection of lymphocytes at these 

 places are foreign to this situation so far as their origin is con- 

 cerned. 



The gills in the ammocoetes are homologous with and extend 

 in the same direction as the gills in elasmobranchs. 



The branchial 'epithelium' does not represent a pure endo- 

 dermal epithelium. This 'epithelium' develops haemopoetic 

 properties in the advanced larva. 



'Epithelium' from the gill arches invades the ciliated epithe- 

 lium of the epipharyngeal ridge and produces placodes. These 

 placodes have a relationship to the gill arches and gill pouches 

 which makes them homologous with the thymus placodes of 



