Whether the fetus is animated at conception or some time later, there is no foundation whatever for the notion that the female is quickened later than the male. As was said before, Aristotle held that the human male fetus is animated at the fortieth day, the female at the ninetieth day, and the old moralists accepted his statement. At the fortieth day, however, no one can differentiate sex unless the microscope is used, and this particular use of the microscope is altogether modern--the knowledge requisite for such use was not in existence sixty years ago. At the twentieth day, with the microscope and a stained specimen, a biologist can recognize whether the primordial ova are present or absent and thus determine sex. Only at the eighty-fourth day can sex now be differentiated without the aid of the microscope, but then the embryo must be dissected: nothing can be told from its external appearance. Sex can first be distinguished by the external appearance only at about the one hundred and twelfth day, the end of the fourth month of gestation. Therefore when Aristotle said the male fetus is animated at the fortieth day, and the female at the eightieth or ninetieth day, he was romancing. The question, then, narrows to this: Is any human fetus animated immediately at conception, or from forty to eighty days after conception? The reason given by the followers of Aristotle for deferring animation is that the vital principle requires organs in the receptive material, but the embryo in the early stages, they say, lacks these organs. This notion, however, as to the lack of organs is altogether erroneous, and the rational soul enters the embryo in the oval stage, immediately after the pronuclei unite: there is organization in that stage of human life sufficient to receive the substantial form or soul. We do not know how long after insemination the pronuclei unite, but the proposition here is that as soon as they unite the human soul enters. Fecundation usually occurs after a menstruation, but not necessarily so; the spermatozoön may live in the tube for seventeen days awaiting the ovum. The human body is made up of billions of microscopic living cells, all of which are derived by fission and differentiation from the two original single germ-cells, the ovum and the spermatozoön. Some nerve-cells have long processes running along the white fibres through the entire length of the body, but they cannot be differentiated except by the microscope. In the body are also various liquids which are not cellular, as water, saliva, tears, urine, blood and lymph plasma, and the gastric, intestinal, and glandular juices, and these are secreted or excreted by the somatic cells. The cells assimilate nutritive material carried to them by the blood, excrete refuse substances, secrete glandular products, and are the media for all human operations below certain acts of the intellect.