Extensive histologic changes of the endometrium are already well under way at the time the fertilized ovum enters the uterine cavity. During estrus the endometrium becomes hyperemic and edematous, while degeneration of a vacuolar type affects the surface epitl'l.elium. In the postestrous period the endometrium becomes markedly thickened, and the uterine glands increase in size and activity. In pregnancy there is further modification of the endometrium, particularly in the carunculae, which serve as a means of attachment for the fetal placenta. The carunculae, more familiarly known as cotyledons, constitute the maternal placenta. The above described changes which provide for the formation of the maternal placenta are initiated by the fertilized ovum and the estrogenic hormones, estrone and progesterone. These hormones, though differing clinically and functionally, are thought to be synergistic. This supposition seems quite logical inasmuch as the cells that form the graafian follicle later give formation to the corpus luteum. The first few days of the fertilized ovum within the uterine cavity may be regarded as difficult ones, largely from the standpoint of nutrition. The secretion from the uterine glands, known as uterine milk, is believed to provide nourishment for a brief period or until contact is made between the fetal and maternal placenta.