ABSTRACT
Measurements have been made of the relative and total numbers of the various types of erythroid cells in the livers of random-bred Swiss foetal mice. It has been shown that the results can be interpreted in terms of two waves of erythropoiesis passing through the liver, one coinciding approximately with the erythropoietin sensitive phase of hepatic erythropoiesis, the other with the insensitive phase.
The mitotic index and thymidine labelling index of early erythroid cells do not change during the period studied, but the labelling index of the late cell types decreases from 30% at 13 days to 15 % at 18 days. This decrease has been interpreted as an increase in the proportion of non-dividing polychromatic erythroblasts.
The cell cycle time of early and late erythroid cells in foetal livers has been determined at 13 and 16 days of age. The cycle time of the early cells is about 
h at both stages of development, but that of late (polychromatic) erythroblasts decreases from 8 h at 13 days to 5–6 h at 16 days.
It has been shown that the results presented are consistent with the change in the rate of increase of hepatogenic red cells which is observed at about the 15th day of foetal life.
