The functions of the corpus luteum.—III. The factors concerned in the development of the mammary gland

Abstract

Phases of Mammary Growth.—The development of the mammary gland from before puberty until the later stages of pregnancy may roughly be divided into four phases:— (a) During the pre-pubertal phase the mammary gland undergoes gradual development, though the gland is still limited to a few ducts in the neighbourhood of the nipple. (b) At puberty (i. e., the first œstrus) growth in the ducts takes place. This phase is well marked in the guinea-pig (Loeb, 17) in the rabbit (Ancel and Bouin, 5), and in teh opossum (Hartmen, 14). In teh absence of pregnancy this occurs at each œstrus. (c) These phases of growth, however, still leave the gland in a rudimentary condition, and after ovulation, correlated with the development of the corpus luteum, the third phase of growth of the mammary gland begins. If pregnancy fails to occur the subsequent removal of the luteal influence results in cessation of mammary growth after a greater or lesser development. The growth of the gland during the luteal phase of the cycle may being rapidly (as in the rabbit) or there may be a hiatus between the development of œstrus and of the luteal phase (as in the guineapig). Even during the luteal phase of the short diœstrous cycle in the non-pregnant animal the gland may be built up sufficiently to admit of a certain amount of secretion (Hammond and Woodman, 13). Where the luteal phase is very pronounced, as in animals such as the rabbit, ferret and dog, the end of the very considerable growth phase is regularly accompanied by the secretion of milk (Ancel and Bouin, 3, 4; Hammond, 12; Marshall and Halnan, 18). (d) During pregnancy the ultimate growth of the mammary gland occurs and this far exceeds the development at any other time. Two phases of growth are found. In the early stages of pregnancy the mammary gland undergoes a degree of development comparable with that found during the luteal phase of the ordinary cycle or during pseudo-pregnancy. In the later stages, however, an entirely new phase of growth begins, which results in greatly increased size of the gland and which culminates in the full development of the gland as found at the end of pregnancy.