DIVISIONS OF ANTENATAL LIFE 9
in the embryonic area of the lilastodermic vesicle, and to end about the close of the sixth week of intrauterine life, a very different pro- cess is going on. There is growth, as in the foetal period ; it is not, however, simple increase, but evolution or development that is the striking feature of the life of the embryo. The lines along which future growth is to take place are nearly all fixed during the embryonic period ; the outstanding phenomenon is the putting up of the scaffold- ing of the future body ; the vitality of the period shows itself in organ formation or organogenesis. As in the history of the rise of a great modern city, there is record of a stage in which the main avenues of traffic are sketched out, and natural obstacles overcome or utilised, to be followed by a period during which growth goes on along the lines of the plan ; so in the story of antenatal life there is the embryonic period, in which the cellular elements are arranged in groups to form organs, to be followed by the foetal, in which these organs simply increase in size, and by their functional activity (in some instances) lead to the growth of the whole organism. This embryonic epoch has a duration of about five weeks, or, if the neofcetal period be included, of about seven weeks. The neofcetal is a sort of transition time during which the placental circulation and economy are being fully established ; in the scheme it has had two interspaces (two weeks) allotted to it. Embryonic life, therefore, like foetal life, ends with a transition time or period of adaptation to new conditions ; in the one case, to the changes consequent upon the organism becoming a placentally nourished one, and in the other to the much more radical changes which atmospheric respiration and gastric digestion entail.
T]ie earliest period of antenatal life is the germinal, and only a small part of it, at its close, comes into the epoch of intrauterine existence. It has a long, a very long primary dual period, during which a semi-independent life of a cellular kind is going on in the male and female reproductive cells, the ovum and the spermatozoon. In the scheme a dividing line indicates this primary dual character of early germinal life. The close of the dual period is marked in the case of the ovum by the phase of maturation, and in that of the sperm by the little known but probably analogous phenomena of spermato- genesis. Then follows the anteconceptional period, during which there is dehiscence of the ovisac in the female with passage of the ovum along the Fallopian tube towards the uterus, and the spermatozoa are deposited in the vagina ; insemination ends this and begins the next period (intraconceptional), in which it may be said that ovular and sperminal life run together in impregnation. Inasmuch as it is known that insemination and impregnation are not of necessity simultaneous, I have thought it well to leave half an interspace (half a week) in the scheme for this event. The rest of germinal life is the unified post- conceptional period, during which the morula mass and the blasto- dermic vesicle are forming, and the first traces of the embryo appearing in the embryonic area. In the scheme, therefore, the dividing line is absent in the postconceptional period, to signify its unified character. Germinal life may be said to pass into embryonic
