No. 607] DIFFERENTIATION IN THE ORGANISM 425 



It is the polyvalent cells which are the source of the 

 wide range of regeneration encountered, particularly in 

 the lower animals. It is astonishing to see how readily 

 students of differentiation and specificity reconcile the 

 extensive regeneration observed in many organisms with 

 the belief in the specificity of the anlages of organs. 

 Driesch has shown that gills excised from an Ascidian 

 can regenerate a whole animal with heart, intestine and 

 stolon. If in the particular case the anlages of the gills 

 and the gills themselves were built of specific cells, the 

 results of the experiment would be inconsistent. How 

 could heart, intestine and stolon regenerate from the gills 

 if the cells of the gills were not endowed with various 

 potencies ; if specific, they would grow only into the same 

 tissue under all conditions. On what other basis could 

 the experiments of Child's be explained, in which cells of 

 a definite segment in the Plauaria will regenerate a head 

 or a tail, according to whether it formed the anterior or 

 the posterior part of the piece, cut out from the worm! 

 The very fact that different specific structures may be 

 regenerated at tlie expense of one coniinoii soiii-ce. as, for 

 example, lieni-t and inlestine from a ^-ill or ei'ytlii-ocytos, 

 granulocytes an<l small lyni] .hoeytes from lienioMasts, 



It is known, indeed, that environment can educe new 

 qualities in the organism, but they usually subsist only 

 while the siDccific conditions are present, and are lost if 

 the organism is transferred to another environment. 

 Such clianges are not specific. The clianues revealed by 

 the freed mesenchymal cells, which result in tlio forniation 

 of mature blood cells, would only then be called speeific, 

 if they were retained by generations of their descendants 

 under different conditions. An indifferent hemoblast 

 within the vessels is soon transformed into an erythro- 

 blast, which shows in its cytoj^lasm the first traces of 

 hemoglobin. Is the erytlirohla-t a definitively s])ecific cell, 

 univalent and no longer eapnhlo of lietei-oi)la-tic differen- 

 tiation in new environment.' Xew environmental condi- 



