Partly for this reason, and partly because it has hitherto been customary to calculate the age according to the system of which His is the most recent advocate, we shall in this chapter give the ages estimated in this manner—i.e., by the age of an embryo will be meant the interval between the first day of the first omitted period and the time at which the embryo is discharged from the uterus. It must be repeated, however, that this method of calculation can only be justified by its general adoption, its readiness of application, and most of all by the fact that no other precise system has yet been proposed. Viewed on its own merits the method is not only imperfect, but is even opposed to many well-established facts.
The First Week.
The process of segmentation in the human ovum has not yet been seen. It is, however, in the highest degree probable that segmentation takes place during the passage of the ovum along the Fallopian tube, and that it is effected in a manner practically identical with that already described in the case of the rabbit.
Neither do we know the length of time taken by the human ovum to travel down the Fallopian tube to the uterus. According to the best authorities this passage "probably occupies not less than eight days in the human subject." In the rabbit we have seen that the uterus is reached on the third day, so that either the process of development must from the first be much slower in the human embryo than in the rabbit, or else the human ovum must enter the uterus in a far more advanced condition than happens in the case of the rabbit. Such evidence as we have points rather in favor of the former alternative.
Second Week.
Of embryos towards the close of the second week of development, a few examples have been described; but there is some doubt whether any of these few can be regarded as perfectly normal.
Reichert's best known instance is an ovum described by Reichert, and estimated to be of about the thirteenth day.
This ovum, which is represented four times the natural size in Figs. 100 and 101, was found in situ in the uterus of'a woman who had committed. suicide. There was a fully formed decidua reflexa, within which was the ovum itself (Figs. 100 and 101), a vesicular body of the shape shown in the figure, measuring 0.2 inches along its greater diameter, and .12 inches from side to side. Of the two sides the one turned towards the uterus (the upper surface in Fig. 101), was more convex than the opposite side, facing the decidua reflexa. The margin of the vesicle, as shown in the figure, was thickly fringed with villi, the largest of which were .07 inches long,
and slightly branched. The villi were absent both from the uterine sur face and from the opposite one, leaving two bare circular patches. In the middle of the uterine patch was a smaller circular spot of a darker color, .05 inches in diameter, and indicated in the figure.
Not the slightest trace of any embryonic structure was discovered; there was no indication of either primitive or medullary grooves. The wall of the vesicle is described as consisting of a single layer of epithelial cells prolonged outwards to form the villi. In the circular patch on the uter ine surface, which is spoken of as the germinal area, a second inner layer of finely granular cells was present. The cavity of the vesicle was occu pied by a gelatinous fluid, traversed by a network of fine fibres, and con taining within it a spherical body.
Ova of somewhat similar appearance, and of apparently about the same age, have been described by Wharton Jones, Breuss, Kollmann, and others; but in none of these cases was any trace of an embryo discovered. _ The chief points that we learn from these other specimens are, first, that the spherical body described by Reichert is made lip of nucleated cells, and is apparently solid and in connection with the germinal area; and, secondly, that it is highly probable that the wall of the vesicle really con sists, not of a single layer, but of two layers, of which the inner one is of the nature of connective tissue, and therefore of mesoblastic origin.
It is not easy to make any satisfactory comparison between these ova and the stages already described as occurring in the rabbit, and the diffi culty is much increased by the doubt we must feel as to whether the ova in question are perfectly normal, or are not to a greater or less extent pathological. As the different ova described appear, however, to agree in the most important points, it is advisable to make such comparisons as is possible between them and the more usual processes of mammalian devel opment.
His considers that the stage reached by these ova is a very early one— but very little older, indeed, than that represented for the rabbit in Fig. 95; and he illustrates his views by the diagrammatic section of Reichert's ovum given above (Fig. 102.) He considers that the outer wall of the vesicle consists of epiblast alone; that hypoblast is present only in the germinal area, where it forms the inner layer described by Reichert; and that the spherical mass of cells is also hypoblast, and will afterwards become hollowed out and expanded to form the yolk-sac.