From the moment of the fusion of the nuclei of the male and female cells, active changes and nuclear divisions are in progress. The egg cell, which is free, travels slowly to the allotted place in the womb or uterus of the mother, and there it settles down in the tissue of the wall and attaches itself. Until it has attached itself firmly to the wall of the uterus, conception proper has not finally taken place, and a fertilized egg cell may be lost through want of a capacity to attach itself to the womb, or through some nervous or other disturbance of the walls of the womb, which throw it off after it has been attached. The distinction between the actual moment of fertilization (or union of the male and female nuclei) and of the final attachment which secures true conception is an important one, though frequently overlooked. Sometimes the failure to conceive a child may not at all be due to lack of fertility and readiness to unite on the part of the egg cell and sperm cell, but may be due to some nervous or other influence on the wall of the uterus, which consequently throws off the ovum before it has firmly settled into its place there. A few days after conception, and when the ovum has attached itself to the proper place, a definite zone of tissue begins to form which, growing and altering with the growth of the tiny developing child (which is now called the embryo), forms a medium of transmission between it and the mother through which pass the substances used and excreted by the embryo in its growth. After fertilization, intense and rapid activity takes place in the nuclei of the cells, first in the united nucleus of egg and sperm cell, and later in the nuclei of all the resulting division cells. The nucleus of the sperm cell is supposed to contain twelve chromosomes which go through a formal rearrangement and mingling with the corresponding chromosomes in the egg cell. As a result of the complete fusion and intermingling of the male and the female factors on fertilization, all the resulting divisions of cells which follow derive their nuclei partly from the male and partly from the female nucleus of the parents. Thus, if it were possible to trace the history of every tissue cell in the body of your child, we should see that each nucleus of all the myriads that compose its structure would ancestrally consist of part of the many sub-divisions of the nuclei of both father and mother. Thus to speak of one side of the body as being male in its inheritance and the other female, is the most unmitigated nonsense, though this idea formed the basis of a recent book.