The Growth of Spontaneous Mammary Carcinomas of C3H/Mza Female Mice as a Function of the Time of Day and of the Degree of Differentiation

The effect of the time of day and of the degree of differentiation on the mitotic activity of spontaneous mammary carcinomas of C3H/Mza female mice was investigated. The animals were kept in standard conditions and killed either at noon or at 8 p. m.; the mitotic activity (mitoses/1000 nuclei) was determined in differentiated and indifferentiated areas of the mammary carcinomas. The differences found in areas of equivalent differentiation in the two times considered were not significant. The differentiated areas had a lower mitotic activity, more so if the differentiation was both architectural and cytologic. This result may indicate that the two characters of progression investigated, i. e. intrinsic growth rate and differentiated evolution of the tumor cells, are not quite independent.