424 



THE AMEBIC AN NATURALIST [Vol. LI 



tors, which from a homogeneous cell material moulded 

 different products. 



On the basis of descriptive histogenetic studies it 

 seemed plausible to admit that environment can modify 

 isolated cells; that the metabolic processes of the cells 

 are the resultant of their physico-chemical constitution 

 plus physico-chemical conditions of the environment (of 

 course hormons, enzymes and so forth are included in the 

 environment) and do not depend exclusively upon their 

 physico-chemical constitution; that different substances 

 arise in the cell-body (hemoglobin, various specific gran- 

 ules) in pol>^alent cells as result of changes, deter- 

 mined by differences in the environment. The exist- 

 ence of cells endowed with various potencies has in conse- 

 quence been largely admitted. The specificity of the 

 various mature blood cells would thus be brought about 

 by factors extrinsic to the stem cells. 



These conclusions are based on facts established by 

 descriptive histogenetic studies. Experimental proofs 

 are beginning to accumulate which soon will leave no 

 doubt of the validity of these conclusions. The existence 

 of polyvalent cells would be proved, if, for example, 

 hemoblasts subjected to various conditions would undergo 

 various differontiation. If stom cell?^ from within the 

 vessels were ti-nii^t'crri'.l into tiic spnccs between the 

 mesencliyitijil cdU and licic ol' developing into 



erythroblnsts, di fft'i-ciitiatcd into .uTannlobla^^ts '(these 

 experiments are under way) the stem cells within the 

 vessels would be proved to be polyvalent. Tlie same 

 applies to other blood cells. As recently sliown, >])leenic 

 follicles, the cells of which normally differentiate into 

 small hanphocytes, if grafted on the cliiek ailantois 

 resolve themselves into nnniefou- liein()l)la>t>. which 

 finally undergo a granuloblast ie <li ffeicntiation and give 

 typical granular leiicocytf-s. T\m< th." results of the 

 liistotieiK-tic .tudie^ l.y ex|.erinient;il method entail the 

 re<-ou-nition in the .-mhryo and in the a.lnlt organism of 

 tissues and ('('lis, which have not been fully differentiated 

 and remain polyvalent. 



