In situ morphology of fetal liver liquid cultures. (A) Cells infected with wild-type hβc retroviruses grown in the presence of growth factors (mIL-3, mGM-CSF, and hEpo). (B) Cells infected with V449E retroviruses grown in the absence of growth factors. (C) Cells infected with FIΔ retroviruses grown in the absence of growth factors. (D) Cells infected with I374N retroviruses grown in the absence of growth factors. Photographs are at 300× original magnification and were taken at day 14 of culture.