By the third month the embryo reaches an average length of three or more inches, and weighs on an average about 2-1/2 ounces. In this month the sex organs of the future baby are rapidly developing, and indeed are rather unduly prominent in proportion to the other parts which enlarge relatively later. Between the third and the fourth month, or often not till a little after the fourth month, the active muscular movements of the embryo’s limbs can be felt by the mother. The experience of this, like the consciousness of the moment of conception, depends very much upon the sensitiveness and delicate balance of the mother’s conscious control of herself. Some are insensitively, though perhaps comfortably, unaware of what is going on in their systems; others are conscious, not of what is properly going on, but of what is going wrong in their systems owing to disease or maladjustment; but there are others who, in perfect health, are yet so acutely sensitive and conscious that they can at will detect, as it were, the condition of their whole organs. Such women as these will sooner feel the active movements of the embryo than those who are less perceptive. As a rule, medical practitioners estimate that about half-way between the date of conception and the date of birth, which should be a full nine calendar months, that is to say about 4-1/2 months from the date of conception, muscular movements of the child are detectable and distinct. In the third month, however, some women are conscious of the most delicate fluttering sensation. By the end of the third month, a definite enlargement of the mother’s body becomes visible, because not only the actual child within her has to be accounted for in the space among her organs, but all the accessory growth of the chamber which accommodates the child in the womb has to find its place, the womb growing rapidly and containing not only the child, but the large amount of fluid by which the child is surrounded, and in which it partly floats. The visible changes in the mother to some extent depend on the proportion of this fluid which develops, some having much more than others, and it is to this rather than to the actual size of the child for the first four or five months that any outward change is due. About the end of the third month the soft and cartilaginous beginnings of the vertebral column begin to harden in various centres, and afterwards the hardening of the bones (or ossification) slowly spreads throughout the whole skeletal system. For some other bones in the body, however, the hardening is not fully completed by the time of birth.