ENDOTHELIUM IN TISSUE CULTURES 41 
reticular outgrowths (Lynch, ’21). I have regarded these retic- 
ular outgrowths as endothelium. They differ in character 
from the reticular outgrowths of ordinary subcutaneous mesen- 
chyme, as can be seen by comparing figures | and 2 of subcutane- 
ous mesenchyme with figures 3 to 12 of endothelium. 
MATERIAL AND METHODS 
The explants were all from the livers of chick embryos of five 
to ten days’ incubation. The cultures were made in the usual 
manner with Locke’s solution (NaCl, 0.9 gram; CaCh, 0.024 
gram; KCl, 0.042 gram; NaHCO;, 0.02 gram; dextrose, 0.5 
gram, H.O, 100 ec.), 80 ec., plus chicken bouillon, 20 ce. The 
chicken bouillon was made according to the usual bacteriological 
method, except that freshly killed chicken was used in the place 
of beef. The hydrogen-ion concentration of the solutions varied 
from pH 6.4 to pH 7.6, but was usually about pH 6.8 to pH 7.2. 
The figures are all from untouched photographs. The cul- 
tures from which they were taken were first subjected to a weak 
solution of janus green or neutral red, or the two in combination, 
until the mitochondria or the granules (and vacuoles) or both 
were stained. The cultures were than fixed with iodine vapor 
by placing a small flake of pure iodine on the bottom of the 
hollow-ground slide. The iodine vapor rapidly tinted the cells 
a beautiful yellow-brown. The mitochondria when stained with 
janus green became a very dark brown (almost black) and the 
neutral red granules became a dark red-brown. ‘These colors 
are very advantageous for photography. 
GENERAL CHARACTERISTICS 
The endothelial cells migrate out from the liver explants onto 
the cover-glass in the form of a loose reticulum of elongated 
cells that are more or less adherent to one another by their ex- 
tremities and processes. ‘The character of this reticulum varies 
considerably in different cultures and in different regions of the 
same culture, as will be seen by comparing figures 3 to 12. Near 
the explant and in the middle of the outgrowth the cells are 
elongated and slender, while at the periphery, where the crowding 
