Placenta, a complete organ, formed partly from the inner walls of the uterus of the parent, and partly from parts of the foetus, by which the latter is nourished during its intra-uterine life. It is, in fact, a specially-modified secreting gland to which the blood-vessels of the foetus act as ducts. The placenta is characteristic of Mammalia; but varies considerably in development in the various subdivisions of the class. The term is by analogy extended to the spongy tissue from which the ovules spring in spermaphytic plants. [Placentation.] This often serves also as a conducting tissue for the growth of the pollen-tube to the micropyle.