DEVELOPMENT OF TIIE HUMAN EMBRYO AND ECETUS.
Concerning the early development of the human embryo, our knowl edge is still in a very unsatisfactory condition. Of the stages passed through during the first fortnight after impregnation we know exceed ingly little, and those occupying the third week are only very imper fectly known to us; but from the end of the third week onwards the various stages of development have been tolerably fully and satisfactorily described.
For reasons stated at the outset, we shall here give a brief account of such early stages as have been described, and endeavor, by the aid of the facts already set forth, to frame something approaching to a consistent account of the development of the human embryo from the ovum.
Estimation of Age of Embryos.
A preliminary difficulty, and one that cannot be satisfactorily disposed of at present, lies in the fact that, after we have obtained an embryo, we have no really trustworthy means of determining its exact age. One of ' the most constant accompaniments of pregnancy is cessation of menstrua tion; and His, one of the greatest authorities on the subject, considers that this fact affords the most trustworthy basis for estimating the age of embryos. He lays down the following rule:' The age of an embryo is the time that has elapsed since the first day of the first omitted period. Thus, supposing the commencement of a period to be due on January 5, and that when this time comes the period is omitted, but some time subsequently, say February 9, an embryo is aborted; then the age of that embryo would be, according to His, the interval between January 5 and February 9—i.e., five weeks.
In arriving at this result His argues in the following manner: The ovum leaves the ovary either at or shortly before the menstrual period; if fertilized, presumably by spermatozoa previously introduced, menstrua tion does not occur; but the changes in the uterine mucous membrane, instead of, as usual, becoming retrogressive, continue to be progressive, and so prepare the uterus for the reception of the ovum. Hence the first
omitted menstrual period corresponds in point of time with the fertiliza tion of the ovum; and hence the age of the embryo may be taken as the time that has elapsed since the first omitted period.
This method of calculation is, however, open to very grave objec tions. We have already seen that there is much reason for think ing that the decidual membrane, which is broken up and dis charged at the menstrual period, is prepared for the reception, not of the ovum which is liberated from the ovary at the time of the period, but of the ovum set free at the last preceding period. The process of fatty degeneration, associated with the break-up and discharge of the decidua, has almost certainly commenced before the occurrence of the period ; and it is almost inconceivable that the mere act of fertilization at the commencement of the Fallopian tube of an ovum which, in all probability will not reach the uterus for at least a week, should be able to arrest the degenerative changes already commenced in the decidua, stop suddenly the menstrual discharge that is on the verge of taking place, and convert the retrogressive changes of the decidua into progressive ones. His's theory also does not accord with the well-estab lished fact that, in order to insure pregnancy, the most favorable time for copulation is shortly after the period, in which case fertilization and the commencement of development of the ovum could hardly be coinci dent with the first omitted period.
For these reasons the theory advocated by Pflfiger and others appears preferable, according to which the decidua discharged at any period is not related to the ovum set free at that period, but to the ovum liberated at the immediately preceding period. On this view, however, we are left ab solutely without means of determining the age of embryos; and although an exact determination is immaterial in the case of later embryos, yet when we are dealing with early ones it is a point of great importance.