40) WARREN H. LEWIS 
cells and wandering cells from the spleen (Foot, 712), bone- 
marrow (Erdmann, *17), lymph nodes (Lewis and Webster, 
21a, ’21b), and thymus (Pappenheimer, 713) seem always to 
migrate out as isolated cells and to remain so. Skeletal muscle 
has been observed by M. R. Lewis (15, ’19), and Lewis and Lewis 
(17 a), and premuscle by Congdon (715). This tissue is usually 
easy to recognize by its elongated multinuclear fibers and pecu- 
liar cytoplasm. Heart muscle has been studied by Burrows 
(10, ’12), Congdon (15), and more especially by Levi (’16b, 
19). Some of the figures of heart muscle shown by the last- 
named investigator correspond to our own unpublished ones, 
but this is not the case with those of Burrows and of Congdon, 
except possibly Congdon’s figure 5. Smooth muscle resembles 
more the ordinary mesenchyme than either skeletal muscle or 
heart muscle and has been observed by Lewis and Lewis (717 b) 
and by M. R. Lewis (’20 a). 
We have been using in a rather loose fashion the terms mes- 
enchyme, connective tissue, and fibroblast to designate the reticu- 
lar radiating outgrowths in tissue cultures, because we were 
not sure of the exact identity of the tissue. I think most in- 
vestigators working with tissue cultures have felt, as we do, that 
in using these terms they were probably dealing with several 
types of cells, namely, mesenchyme (including reticulum), fibro- 
blasts, and other types of fixed connective-tissue cells, endothe- 
lium, and mesothelium. The literature is full of figures of these 
mesenchymal tissues, and among them are some which are prob- 
ably endothelium. Most explants contain capillaries with 
endothelial cells and in many cultures these cells probably mi- 
grate out, although they may not be recognized as endothelium. 
The present study is devoted to a consideration of the endo- 
thelial cells that migrate out from the explants of embryonic 
chick liver. It is well known that the liver, after ninety-six 
hours of incubation contains but two types of cells, the liver cells 
proper and the endothelium of the sinusoids (Minot, ’00). It 
seemed to offer, therefore, an excellent opportunity for the study 
of endothelial outgrowths, as no difficulty should be encoun- 
tered in distinguishing the liver-cell membrane from the loose 
