Memoirs: The Growth-Changes in the Mammary Apparatus of Dasyurus and the Relation of the Corpora Lutea thereto

Formation of the Gland.--(1) The teat anlagen arise from a proliferation of the cells of the Malpighian layer of the epidermis. (2) The mammary gland anlage arises as a solid cellular outgrowth from the follicles of especially strong hairs on the primitive teat anlage. (3) The hairs ou the teat anlage are equal in number to the main milk-ducts in the adult teat. (4) The proximal solid part of the mammary gland outgrowth gives rise to the main milk-duct and the distal, hollow branches to the mammary tubules of the adult. The Adult Gland.--(1) The fully grown gland in repose consists of six main ducts, with a lining epithelium several cells deep, opening on the teat. The ducts lead down into a large number of ramified twisted tubules whose walls are two cells thick. (2) It is the seat of continuous, but very slow, secretory activity. Changes prior to Ovulation.--There is an increase in the size and activity of both the sebaceous and sweat-glnnds of the pouch area, but very little, if any, growth in the mammary gland. Changes during Pregnancy.--(1) There is a large increase of mammary gland tissue at this period, resting upon the initotic division of its epithelial cells, and resulting in the formation of a great number of true alveoli with single-layered walls and a secretion of lymph. (2) The sebaceous and sweat-glands of the pouch area also hypertrophy and become active, the secretion of the latter appearing as drops of a reddish fluid. (3) The greater part (practically the whole) of the growth in actual gland-tissue occurs before the attachment of the embryo. Changes after Parturition.--(1) The secretion of milk does not commence until about twenty-four hours after the birth of the young. (2) Such secretion results from a purely secretory activity of the cells of the glandular epithelium, and is not connected with necrobiosis of either cytoplasm or nucleus. Changes wheu Ovulation is not succeeded by Pregnancy.--(1) A large growth of the mammary gland-tissue occurs, due to the mitotic divisions of its epithelial cells, and it results in the formation of true alveoli wioh single-layered walls. A substance which resembles milk is secreted in small quantities in these alveoli. (2) The sebaceous and sweat-glands of the pouch area hypertrophy and become active. (3) The above changes are very similar to, indeed indistinguishable from, those taking place during pregnancy. Stimulus causing the growth of the Mammary Glands.--(1) The stimulus causing the hypertrophy of the mammary gland at any time is not a nervous one, but of the nature of an internal secretion carried by the blood. (2) The seat of origin of the stimulus produciug this growth at, puberty, during heat, in virgins that have given milk, and in Dasyurus after ovulation not succeeded by pregnancy is neither in the foetus nor in the placenta. (3) The presence of a functional ovary is necessary for the growth of the gland at all times. (4) The Corpus luteum is a ductless g-land producing an internal secretion which is responsible for the attachment of the embryo to the wall of the uterus. (5) (A) In Dasyurus, in the non-pregnant as well as in the pregnant animals, the formation and growth of the corpora lutea are intimately connected with the growth of the mammary gland. In the non-pregnant animals the secretion of the corpora lutea appears to be the only assignable cause of the growth of the gland. (B) In the rabbit, (a) the presence of an ovary (with Corpora lutea) is necessary for the growth of the mammary gland; (b) the growth of the mammary gland follows on the experimentally induced production of Corpora lutea. The general conclusion supported by the above evidence is that the Corpus luteum is a ductless gland producing a secretion which is the inciting cause of the growth of the mammary gland. Cause of Milk Secretion.--There is at present no satisfactory theory of the causation of milk secretion. That of the removal by parturition of an inhibitory stimulus originating in the placenta or foetus does not meet all cases.