“…Recently it has been claimed that a rapid proliferation (or growth) of cells may be correlated with a high content of ribonucleic acid (Brachet, 1941;Thorell, 1947) and/or of alkaline phosphatase (Danielli, 1946;Moog, 1946), 10 MR B. I. BALINSKY ON THE DEVELOPMENTAL PROCESSES IN both of which substances may be demonstrated by cytochemical methods. The alkaline phosphatase is of special interest in this connection, as it has a specific relation to the differentiation of hairs (my attention has been drawn to this fact by Miss M. H. Hardy, 1949), and is also present in fully differentiated milk-gland tissue (Folley, 1940). The content of the ribonucleic acid in different stages of growth and differentiation of embryonic structures is of special interest as it has been suggested that it may be playing a very prominent role in the determination of tissues and cells (Brachet, 1944;Darlington, 1944;Wright, 1945;Spiegelman, 1948).…”