ABSTRACT
The hind limb-bud mesenchyme of chick embryos 
days old was cultured in Eagle’s Minimum Essential Medium supplemented with both horse serum and fresh chick embryo extract. Whereas no differences are seen at the light-microscope level, at the electron-microscope level the chondroblasts differentiated in tissue culture are noticeably different from those differentiated in vivo, particularly in the possession of some cytoplasmic fibrils and vacuoles. It is proposed that the secretion of the extracellular matrix alone is not sufficient to account for the pattern of cellular arrangement in a cartilaginous condensation.
