304 VERA DANCHAKOFF 



notice that the myeloid metaplasis is produced by different 

 causes. The toxines of various bacteria, the specific products 

 of metaboHsm of mahgnant tumors, finally, inorganic chronic 

 intoxications may incite an extensive myeloid metaplasis. It 

 is difficult to conceive in such qualitatively different agents a 

 specific stimulating influence on the stem cells. The response 

 to the action of these factors is specific in so far as it is exhib- 

 ited by a certain kind of tissue (even not of cells). The stimu- 

 lus itself may largely vary. The cell, being understood as a 

 complex group of receptors, may offer adequate receptors to 

 different stimuli or antigenes. A similar example of stimulation 

 to proliferation by different agents may be found in the phe- 

 nomena of fertilization. The specific or usual stimulus in the 

 form of the spermatozoon may be replaced by other chemical 

 stimuli, which may find adequate receptors in the egg cell and 

 incite therefore molecular changes, which are followed by pro- 

 liferation. 



If more than occasional coincidence is to be seen in the regular 

 connection of the appearance of different antibodies and the 

 myeloid metaplasis after infections, the specific antibodies may 

 consist of substances derived from the proliferation and differ- 

 entiation activities of the hematopoietic tissue. If so, a mere 

 stimulation of the hematopoietic tissue would suffice for strength- 

 ening or developing immunity; the production of a large amount 

 of antibodies would follow as a result of stimulation plus speci- 

 fic action of the antigene. Otherwise how could be explained the 

 development of immunity against heteroplastic grafting by intro- 

 duction in the organism of an emulsion of various tissue (Da 

 Fano)? This intervention, similarly to the experiments de- 

 scribed, must have also stimulated the hematopoietic tissue. 



It is too early now to attempt to draw more definite conclusions 

 concerning the specific functions of the hematopoietic tissue. 

 Confronted with the simultaneous development of widespread 

 changes in the mesenchyme, which occur after the appearance in 

 the organism of antigen-like substances and usually followed 

 by a production of specific antibodies, one may find it natural 

 to think of the mesenchyme and its differentiation products as of 

 an organ in close relation to the production of immune bodies. 



