[Comparative investigation of the growth of rat and mouse embryos].

The growth of rat and mouse embryos follows a different course. Volume and weight studies have shown that after 65% of the gestation period has elapsed, the mouse embryo is larger and heavier than the rat embryo. At birth, however, the rat is about 4 times as heavy as the mouse. The mouse has a more or less uniform growth over the entire gestation period, whereas the chief growth of the rat occurs in the last third of the gestation period. The rapid growth of the rat during this time does not result from an increment in water content; the dry matter undergoes an increase nearly proportional to that of the total body weight.The cells of the 17-day mouse embryo are somewhat smaller than those of the 19-day rat embryo, but the difference is small. On the other hand, the rate of mitosis in the 19-day rat is 20% higher than that of the comparable mouse embryo, in accordance with the faster growth which becomes established at this time in the rat.In contrast to growth, certain of the processes of differentiation occur earlier in the rat than in the mouse. This holds true, for example, for the formation of the crypts in the small intestine and the hair anlagen. The ossification of the cartilaginous skeleton begins at about the same time in the two rodents.