1. The general course of prenatal growth in the mouse, the guinea pig, and the chick can be expressed by straight line relations between the logarithms of the weight and age only when age is counted from the beginning of the embryo proper. 2. This is interpreted as showing that the manner of growth before the beginning of the embryo proper is essentially different from that after this time. 3. The velocity constants for the animals mentioned are similar; the major differences in their curves depend on the amount of tissue involved in the first organization of the embryo proper and in the length of prenatal life. 4. Growth of different animals may be compared more accurately if, instead of either birth age or conception age, embryo age is used.