If this view is correct, it is clear that in the human embryo the allan tois is formed unusually early, and in an altogether exceptional manner. We may clearly connect this "precocious" development of the allantois with the "precocious" appearance of the vascular layer of mesoblast lining the blastodermic vesicle in the stage represented by Reichert's ovum; and we may perhaps regard both features, in so far as they are exceptional, as examples of the tendency to abbreviation or shortening of the processes of development, which is a feature so constantly encoun tered by the student of embryology. The establishment of a vascular connection between the embryo and the chorion, and so indirectly with the mother, is, as we have seen, the characteristic feature of mammalian development, and, therefore, we need not wonder at finding in the most highly developed of all mammals this feature thrown back to an earlier stage than that at which it originally appeared, and hurried on prema turely, even at the expense (as it would seem) of the embryo itself, whose development is unusually retarded.
The series of figures given above indicate also the supposed stages in the development of the amnion, the sole peculiarity in which is that the head-fold—always the most prominent portion—here forms, with the side folds, the whole amnion, there being no tail-fold developed at all. After completion of the amnion, the villi, previously absent over the ger minal area, extend all over the ovum. Cf. Fig. 109, C.
Summarizing what we know about the processes of development in the first fortnight, it would appear probable that the ovum—fertilized in the upper part of the Fallopian tube—travels slowly down towards the uterus, which it reaches about the eighth day. While in the tube it almost cer tainly undergoes segmentation in the usual mammalian manner, but does not increase greatly in size; according to Allen Thomson, " its diameter on arriving in the cavity of the uterus does not probably surpass one hundredth (0.25 mm.), or at most one-eightieth of an inch." After en tering the uterus it probably increases rapidly in size. It very early de velops villi on its surface, and is completely enclosed in the decidua reflexa, at any rate, by the thirteenth day. In the development of the ovum the most noteworthy features appear to be the very early establishment of a lining of vascular mesoblast to the blastodermic vesicle, the very early appearance and peculiar mode of formation of the allantois, and the curi ously late appearance of the embryo itself.
Krause's the allantois it ought to be mentioned here that a human embryo of a very much later stage than those we have just considered—i.e.,about the fourth week—has been described by Krause,
in which there was no allantoic stalk connecting the embryo and chorion together, but the allantois hung down as a bag from the hinder end of thee embryo, very much as shown in Fig. 98, 4. As this is at present an isolated exception to the general rule concerning the allantoic balk in human embryos, it is perhaps permissible to regard it merely as an abnormality, in which case it may be viewed in the light of a reversion to the primitive mode of development of the allantois.
Third Week.
Of embryos belonging to the first half of the third week, only a limited number have been accurately described and figured; but towards the close of the week specimens become far more abundant, and from this point onward our knowledge of the development of the human embryo is in a tolerably satisfactory condition.
Fig. 110 shows the condition of the embryo at an age between the fifteenth and eighteenth days, as described and figured by Coate. The whole ovum measures .63 inches along its greater diameter, and is covered exteriorly with short, slightly branched villi. The embryo is attached to the inner side of the choriou by the short allantoic stalk seen at the left-hand end of the figure, A.s. The embryo itself is .017 inches in length, and is invested—not closely, but at some little distance—by the amnion. The head end of the embryo is completely raised above the yolk-sac, but the body is still so widely connected with the yolk-sac, that one can hardly speak of a distinct yolk-stalk.. The hinder end of the embryo is bent up wards rather strongly—a very characteristic feature of the early human embryo, and one which is very probably to be ascribed, at any rate in great part, to the peculiarity already noticed concerning the allantois.
In the neck three thickenings—the visceral arches—are visible on each side, but the clefts between them have not yet been completed; below the neck, in the angle between the embryo and the yolk-sac, is the heart, a large tube twisted into an S shape. Blood-vessels are visible on the yolk-sac--which have a diameter of .1 inches—and also in the allantoic stalk, whence they pass into the chorion, the inner layer of which is vas cnlar all round the ovum, though the blood-vessels do not as yet pass into the villi.
The middle portion of the embryo is clearly divided into protovertebne; but there are no traces whatever of limbs, or of either eyes or ears.