150 



in lu'lcroj^ciiic scniin obliiincd IVoiii yoim^ :iiiiin;ils was 

 imich ^rcnlcr llian in lu'lcroncnjc scnini IVom :)l(l('r 

 animals II was (licrcrorc assumed lliat chicken librohlasls 

 could be used also as a I'ea^eni i'oi' (lie deleclion of ll\e 

 modilicalioiis l)i-ouj«ld aboul l)> a^e in llie hlood oj olliei' 

 animals 



ir (he conceulralion ol Ihe lielero^enic sera was increased 

 beyond a certain <«rade. the inliibilin^ action of Ihe medium 

 ra|)i(lly became marked and ollen no •^rowtli was ;)bserve<l 

 when Ihe conci'nlralion i-eached ."50 1.") per cenl. Sei'a were 

 obtained from cats and doj^s. Ilie inhibilini; iid'luence ol' 

 heterogenic serum was found lo \ary in direct ratio to the 

 age of tlie animal from wliich it was ol)tained 



Many attempts ha\i' been made 1)\' (larrel to lind 

 soni' ])hysical or chemical jM'operties of the various sera 

 fr(jm yonng and old animals which would corres])()nd to 

 their biological |)roi)erlies of increasing or decreasing the 

 rate of growlli of fibi'oblasts. I-'rom this point of view 1 

 have together with Carrel*) made (piantitative chemical 

 investigations of the \ai'ious st'i'a and at the same time 

 their action on sti'ains of fibroblasts was tested. 



The relati^■e amounts of globulins and albumins were 

 measured l)y means of the H o 1) e r t s o n's I'efractonietric 

 method. At the same time the amount ol total nitrogen and 

 the amino acids and amino nitrogen SorenscTi. \' a n 

 Slyke was determined of the \ai'ious |)r<)teins 



The I'esults of these exj)ei'iments wei"cu that with the 

 increasing age of the animal, the niti'ogenous substances 

 increased, also the inhibiting effect of the sera and \- i c e 

 vers a. 



It was found recently l)> (larri'l and blbeling '^ that 

 the action of serum on homologous fibi-oblasts was not due 

 to one inhibiting substance only, but to the cond)ined action 

 of two substances, one activating and the other inhibitijig. 



*) Unpublistied experiments. 



