Although the exact mode of embryoiiic development of blood vascular endothelium has not been definitely established for all classes of vertebrates, there is a general agreement that the growth of the vascular system, after a brief period of primary differentiation, is accomplished by the sending out of endothelial sprouts which anastomose with neighboring sprouts to form a primitive capillary network. The formation of capillary sprouts from preexisting endothelium has been observed directly in the living animal in the transparent tails of amphibian larvae by many investigatoix (Golubew, 1869; Arnold, 1871; Rouget, 1873; Clark,'09,'18). The transformation of the primitive indifferent vascular plexus, following the onset of circulation, into arteries and veins has been described by Thoma (1893), Mall ('06), Evans ('09), Clark ('18) and many others.

While there is a division into two stages in the development of the vascular system-a stage of primary histogenesis for which hereditary factors are largely responsible and a second stage which is chiefly controlled by circulatory conditions and tissue metabolism-there is an overlapping of the two processes in the embryo. Thus Stockard ('15) has shown that endothelial sprouting begins before differentiation of endothelium is everywhere complete and Lewis ('31) has demonstrated by his studies of capillaries in tissue cultures that