NVESTIGATIONS on causative factors in the I genesis of mammary carcinoma in animals, especially in mice, have made it evident that several factors are involved in its development. Somc years ago it was found that the so-called milk factor, an agent that is transmitted by the mother's milk, plays an important part in the development o€ mammary carcinoma in mice. There is no evidence that a milk factor is present in other species or in human beings. Investigations in recent years have made it clear, however, that the milk factor is not necessary for the development of mammary cancer i n mice. T h e milk factor is only an accelerator and intensifier and not a causative factor.
If the milk factor has only an accelerating effect, then the question arises, what is the primary factor in the genesis of mammary cancer?
If we study the development of mammary cancer in strains of mice without milk factor, then it can be shown that mammary cancer develops only in mice that have been pregnant. X'irgin females show n o or only a very few mammary tumors. This would lead to the conclusion that pregnancy is a factor of importance in the etiology of these tumors.
Pregnancy is a very complex process; it may be presumed, however, that the hormonal changes during pregnancy in the mammary gland constitute the essential factor.
It is an established fact that a developed mammary gland is conditional for the occurrence of mammary carcinoma. T h e development of the mammary gland, in turn, depends on the effect of hormones. Hormones of the ovary and the pituitarv gland are involved both in the growth and in the function of thc mammary $and. T h e developmcnt of the mammary gland during pregnancy greatly exceeds that seen in nonpregnant mice. T h e conclusion would seem to be warranted that it is this increased development of the mammary From the Nethcrlands Cancer Institute,