122 



that it is spread out. Bii( after a little practice this can 

 he aeeoinplished. 



During the cultivation ol cpillu'lial cells in vitro, con- 

 tarninaliou with t'ihroblasls may very easily occur. Some- 

 times a small piece of tissue from the embryonic tissue 

 Juice is accidenlly carried over; sometimes it grows out 

 a little distanee from the epithelial tissue, but at other 

 times fibroblasts can be seen growing in the perijjhery 

 of the epithelium. The cultures have to be examined 

 carefully and if contaminated with fibroblasts they must 

 be kept separately for observation for quite some time. 

 It sometimes ha])i)ens that the culture of epithelium 

 commences to grow at a suspiciously quick rate and the 

 whole appearance of the culture has changed to a fibro- 

 blasl-like culture; in such cases we doubtlessly have had a 

 contamination with fibroblasts wdiich has escaped notice. 

 The contamination is mostly foiuid when the epithelium is 

 allowed to grow embedded in the clot. The explanation is 

 quite natural. When the tissue fragment is placed in the 

 fluid plasma and a drop of tissue-juice has been added, 

 the droj) is then stirred up with the knife and mixed. If 

 a small piece of tissue from the tissue juice should have 

 escaped our notice and should be brought to the plasma 

 with the juice, it will usually agglutinate to the original 

 tissue fragment, and when coagulation takes place these 

 two pieces will grow logethei* and the error usually is not 

 detected befor(> Hie connective tissue cells appear every- 

 where and the culture must i)e discarded. This has happened 

 rather often; usually the tissue which causes the contamina- 

 tion can scarcely be seen in the juice; it is often a tiny 

 jelly-like substance of practically the same refractivity as 

 the juice and can ])r loiiud floating in the u|)per part of 

 the fluid extract. 



There are tw^o ways of avoiding this error; either to 

 prepare the embryonic tissue juice by grinding up the 

 embryos with sand or kieselguhr or by using a Buch- 



