The active egg cell, which is ready for fertilization, is produced in one or other of the two ovaries, which lie internally and cannot be touched or reached in any way without operating upon the mother; they have no direct contact with the outer world. These two ovaries each communicate with the central chamber, which is called the womb or uterus and this is a strong muscular organ, into the walls of which the attachment of the minute embryo fastens, and within this chamber the growing embryo gradually fills the space reserved for it. The womb or uterus has a connection with the outer world through the lower mouth called the os, which opens into the vaginal channel. This os or mouth with its rounded lip can just be felt at the end of the vaginal channel. Fertilization consists in the actual penetration of the egg cell by the male sperm, the nuclei of which unite. As I have elsewhere described (_Married Love_, Chap. V) the numbers of male sperm provided in any act of union outnumber by millions those actually required, because for each single fertilization one egg cell combines with one sperm cell. The egg cell or ovum is very large in comparison with a single sperm; nevertheless it is itself a minute, almost invisible protoplasmic speck, measuring rather less than 1/120th of an inch in diameter, and roughly spherical in its shape--a minute pellet of jelly-like protoplasm with a concentrated centre or nucleus. The single sperm which unites with it is a still more minute fleck, and is little more than a nucleus with a film of protoplasm round it, and a long cilium or hair-like continuation which it lashes to and fro, and thus propels itself or swims towards the egg cell. Judging by analogy, it leaves this tail outside the egg cell on the mutual fusion. The nucleus of the sperm and of the egg unite in a very complex and precise manner. In other organisms, and probably also in human beings, the entry of a single sperm to the egg cell shuts out the possibility of other sperms fusing with them, because directly it has been fertilized, the egg cell exudes a film of substance which antagonizes the other sperms, and which ultimately forms a filmy skin around itself.