It is but natural that investigators in the field of tissue culture should have had and should still have more or less difficulty in identifying the various types of cells that grow out from explants of embryonic tissues: Most explants contain several types of cells that migrate out into the medium, some of which are comparatively easy to identify. Kerve fibers from the central nervous system have been observed by Harrison ('07,'lo), Ingebritsen ('13)'and Levi ('16 a), and those from the sympathetic system by Lewis and Lewis ('12 a), and Matsumoto ('20). They are always quite characteristic. Endodermal membranes from the cells lining the alimentary tract and allantois were not at first identified as such (Lewis and Lewis,'11; Lambert,'la), but later their origin was established (Lewis and Lewis,'12 b) and their recognition is now simple. Ectoderm from the skin (Lambert,'12) and the amnion (Lewis and Lewis,'12 b) grom out in the form of a membrane or sheet, as does also the pigmented epithelium from the retina (Luna,'17, fig. 1; Smith,'20). In the frog, likewise, the epithelial cells grow out as menibrants (Uhlenhuth,'14,'15; Matsomoto,'18). Liver rells form membranes similar to the endoderm and are easily identified (Lynch,'21). The thyroid-gland cclls grow out either as tubules or membranes (Carrel and Burrows,'1 la,'1 lb). Renal epithelium from the tubules likewise grows out in the form of membranes or sheets (Lewis and T, ewis,'12 b) and sometimes as tubules (Reinhoffl). Blood-