On the basis of material comprising 10 developmental stages of man in 35 embryonic series, the authors tried to determine the critical developmental period of the blood vessel wall. This period falls between the 5th and 16th weeks of intrauterine life, that is in the period of the formation of the basic wall structure characteristic of the particular vessel type (i. e. the elastic and muscular arteries, large veins, limb veins, and lymphatics). In limbs the differences between the superficial and deep veins as regards the vessel wall development, were disclosed.