GENERAL ACCOUNT OF MAMMALIAN DEVELOPMENT.
The earliest stages in the development of the fertilized human ovum have not yet been seen at all, and several of the later stages are only very Inasmuch as a general knowledge of these early and intermediate stages is absolutely essential to the proper understanding of the structure of such early human embryos as have been described, a short general account of the leading features in the development of mam malian animals will be useful here, while the development of the human embryo itself will be dealt with later.
Segmentation of Ovum.
The changes that immediately follow impregnation are best known to us through the researches of Bischoff and Van Beneden, as they occur in the rabbit. The ovum is fertilized in the upper part of the Fallopian tube, down which it travels slowly to the uterus, which it reaches in three days. During its passage it is still invested by the zona radiata, and receives, in addition, from the walls of the tube, an outer albuminous envelope. The changes that are undergone by the ovum during its pas sage down the Fallopian tube are commonly spoken of as the segmenta tion of the ovum, and are of the following nature: A few hours after fertilization is effected the whole ovum divides into two very nearly equal portions (Fig. 91, u); a little later each of the two divide again, and then each of the four; so that we get, in place of the original single sphere (Fig. 91, A) eight spherical bodies,of which four are slightly larger than the other four (Fig. 91, D). Each of the eight again dividing, we get sixteen, of which the eight larger ones—which we shall speak of as epiblast cells—become arranged round and partially enclose the eight smaller or hypoblast cells. Both sets of cells go on multiplying, and at about the end of the third day after impregnation, when the ovum passes from the Fallopian tube into the uterus, it has the structure shown in Fig. 92, A. It is still spherical, .3 inch in diameter, and very lit tle, if at all, larger than at the time of escape from the Graafian follicle; but instead of being one single cell, it consists of an outer layer of epi blast cells, almost completely surrounding a central mass of hypoblast cells.
This phenomenon of segmentation, always the first process in the de velopment of an egg, is clearly a process of cell multiplication by fission; and it is almost certain that the nucleus plays the same part here that it does in ordinary cell division—i.e., that before the whole ovum divides into two the nucleus divides first, and that at every subsequent division the nucleus divides before the cell itself, so that the nuclei of the cells shown in Fig. 92 are derived by fission from that of the fertilized ovum.
If we consider that the ovum is a single cell, and that from that single cell the embryo, with its various tissues, epidermis, muscle, nerves, etc., themselves all composed of cells, has to be derived, we shall not be sur prised to find that the very first thing the ovum does in developing is to give rise by fission to a heap of cells—to convert itself from a unicellular to a multicellular condition.
The Blastedermic Vesicle.
having reached the uterus, the epiblast cells first grow over the hypo blast cells so as to enclose them completely, and then a narrow cavity, crescentic in section, appears between the epiblast and hypoblast (Fig. 92, a), extending nearly, but not quite, all round. The epiblast, and conse quently the whole ovum, now grows very rapidly, and during the fourth day acquires the appearance shown in Fig. 93. It is now a thin-walled spherical sac, about one inch in diameter—the so-called blastodermic vesicle. Its wall consists of a thin layer of flattened epiblast cells, and attached to its inner surface at one part is a lenticular mass formed by the hypoblast cells. If the ovum be looked on from above, this patch of hypoblast will give rise to an opaque circular spot—the embryonic area.