(1) Data on the embryonic growth of several organs and of many different types of tissue in the mouse and rat are presented. (2) The concept developmental age is introduced, because it leads to an age estimate that is better correlated with the developmental stage. (3) The general growth pattern of the organs and tissues studied agrees with that found for the whole animal, consisting of phases with exponential growth separated by breaks. In the majority of the organs the breaks occur at the same time as in the whole animal. (4) The decrease in the growth rate at the end of phase E II is definitely smaller in the majority of the organs than in the whole animal. (5) Breaks occurring at other times than those found in the whole animal seem to take place in a few organs, and in the mouse at the embryonic age of about 17 days in a large number of organs.