121 



should happen lo np|)oar in the plasma, il is very seldom 

 that il is ill jusl the saiiu' spol as the IVaj^iiicnl and Ihc 

 conlaiiiinatioii can he dcU'clcd in lime. We lia\{> been cul- 

 li\alin^ epillielium in liial way and haxc coxcred il wiih 

 a liny di'op of Ringers scjlulion instead of lissue juice 

 and Ihe numl)er of fibroblasl-conlaminaled cullur(>s was 

 exlri'mely low. 



The piH'caulions menlioned here show one Ihinj^ which 

 liad nol been I'ealized bet'oi-e or laken into consideralion. 

 namely, liow ol'len a conlaminalion with I'oi'eign cells is 

 possible. II is (piile nalurail.x noliced here in Ihc cxpcri- 

 mcnls on Ihe culli\alion ol' epilhelium for long periods 

 of lime The epilhelium grows (piiie diffcrenlly from the 

 fibroblasts, and it is lliei-efoi'e wry easy to sec when con- 

 nective lissue cells api)eai' in Ihe cullurc. On the other 

 hand, il is imjiossible to discover in cultures of fil^roblasts 

 when new connective lissue cells accidentally are added 

 lo the lissue fragment. Therefore, il is absolntely urgent 

 lo lake strict precautions in working with jjiu'c cultures 

 in oi'der lo avoid disastrous errors. 



In ordei to settle the (juestion of dedifferentiation of Ihe 

 epilhelial cells in vitro a series of experiments was made 

 in which ej)ilhelium and connective tissue were cultivated 

 side by side in the same culture medium. There was always 

 a sufficiently striking morphological difference hetween the 

 two cultures, the connective tissue forming a typical network 

 of cells, and the epithelium growing in compact masses 

 with the cells in close contact with each other (Fig. 19). 

 These cultures were stained with methylene hlue. It was 

 impossible then to see if there was any amalgamation 

 between the epithelial cells and the fibroblasts when the 

 cells of the two cultures were intermingled. Then it was 

 essayed lo slain mixed cullures with Van (1 i e s o n"s slain, 

 but no differential staining resulted from il. In other experi- 

 ments the Iwo sli-ains: 2 'A monlhs old ei)ithelium and 

 10 3^ears old conneclive lissue. were allowed to grow 



