RESPIRATORY NET IN ALLANTOIS OF CHICK 413 



ent cells. The localization of the ebithelial like cells in this par- 

 ticular case might however offer more or less evidence of their 

 ectodermal origin. Epithelial-like membranes, disregarding their 

 origin, are usually formed on free surfaces or around lumina, 

 and the existence of an epithelial-like membrane in the midst 

 of mesodermal structures must strongly suggest its secondary 

 development at the expense of a structure, formerly otherwise 

 situated. 



The identification of the subvascular epithelial-like cells as 

 ectodermal can be established by the study of their histogenesis, 

 but this method must be applied exhaustively. A gap in obser- 

 vation leads often to a wrong conclusion, and the lack of stages, 

 which showed a growth of the capillary net through the ecto- 

 derm led Fiillborn to the conclusion of the mesodermal origin of 

 the subvascular epithelial-like cells. 



A rehable method of identification of cells and tissues can be 

 found in the study of their potencies for differentiation. If two 

 kinds of differently constituted seeds cannot be distinguished, 

 let them grow under equal environmental conditions ; their prod- 

 ucts of development will be different, if the seeds were. In a 

 previous paper ^ concerning the small cortical thymic cells, the 

 latter were identified as true lymphoid elements on the basis of 

 this method. This conclusion was drawn from the experimental 

 result that the stemcells of the small cortical thymic cells under 

 definite conditions differentiated into granular leucocytes. The 

 structural characters of the cortical thymic cells and those of 

 their stemcells long seemed insufficient for their final identifica- 

 tion. The recourse to a study of the potencies of the stemcells 

 in the thymus served to establish definitely their identity as 

 true hemoblasts ; hence the small cortical thymic cells, as deriva- 

 tives of hemoblasts, are not disguised epithelial cells of ento- 

 dermal origin, but true small lymphocytes. 



The same method can be applied to the identification of the 

 subvascular epithelial-like cells in the allantois. The recognition 



" The differentiation of cells as a criterion for cell identification, considered 

 in relation to the small cortical cells of the thymus. The Jour, of Experimental 

 Medicine, 1916, vol. 24, p. 87. 



