Against this interpretation it must be pointed out (1• that the stage in question is in the rabbit a very early one, and it would follow that, except in its much greater size, the human ovum on the thirteenth day has ad vanced no further than the rabbit's of the third day, which, considering the usual rapidity of the early embryonic processes, would be at least very extraordinary; (2) that the evidence is very strong—indeed, practically conclusive—for thinking that the wall of the vesicle has, in addition to the epiblast layer, an inner connective-tissue lining, which is even de scribed by some observers as vascular, and which must be of mesoblastic origin. It appears, therefore, certain that the stage reached is a consider ably later one than supposed by His; and it is also clear that, if normal, it does not exactly correspond to any definite stage in the development of the rabbit.
The human ovum, indeed, would appear to be peculiar (1) in the un usually early or " precocious" development of a layer of vascular meso blast lining the blastodermic vesicle; and (2) in the very exceptionally late appearance of definite rudiments of the embryo itself. It is extremely important that no opportunities of determining whether the latter feature is normal or pathological should be lost in future.
Hils Embryo E.—The youngest human ovum containing a distinct em bryo is one obtained by His in 1869, and carefully described by him, and is at present deposited in the Anatomical Museum at Basle. This em bryo, which is considered by His to be of about the fourteenth day, is rep resented magnified twenty diameters, in Fig. 106. The whole ovum is an oval vesicle, measuring along its greater diameter .33 inches, and along its lesser .22 inches, and covered all over with branched villi. The con tained embryo is .08 inches in length, and attached at its hinder end by a short thick stalk .2 inches long to the inner surface of the vesicle. The embryo is separated by a very slight constriction—most marked at its anterior end—from the yolk-sac (Fig. 106, I, s.), which measures .09 by .05 inches. Covering over the embryo, but at a short distance from it, is a membranous fold A, which is clearly the true amnion. The embryo itself presents on its dorsal surface a medullary groove, bounded by two promin ent medullary folds; the only other organ visible is a slightly prominent fold between the embryo and yolk-sac (Fig. 106, H), probably the rudiment of the heart; two vessels arising close to this were traced over the yolk-sac.
Other embryos of apparently about the same age have been described by His, Allen Thomson, and others, which agree in their main features with that just noticed, and differ principally in being rather further ad vanced—the constriction between embryo and yolk-sac being more marked, the embryo itself being rather larger, and the medullary groove both deeper and longer.
On comparing these embryos with the corresponding stages of the rab bit, the most marked difference is seen to lie in the fact that the human embryo is already at this very early period connected with the chorion by a stalk, while in the rabbit this connection is not acquired till considera bly later. This stalk is clearly the allantois, so that the difference might be expressed by saying that the allantois develops earlier in the human embryo than in the ordinary mammal. But this is not all. Not only does the allantois develop earlier, it also develops in a totally different manner. Usually among mammals the allantois arises, as we have seen above, as a holloy; saccular outgrowth from the alimentary canal, which grows to and becomes connected with the chorion. In the human embryo, however, the allantois appears, from the researches of His, not to arise as an outgrowth from the alimentary canal at all, and, indeed, never to pass through the saccular stage shown in Figs. 100 and 101, but to be present from the very earliest period as a stalk connecting the hinder end of the embryo with the chorion.
The following series of diagrams, copied from His, will make this clear, and will show how the stage now being considered may be derived and probably is actually arrived at, from the stage represented by Reichert's ovum. The figures represent a series of diagrammatic longitudinal sec tions through ova at successive stages of development. Fig. 102, as we have seen, represents Reichert's ovum, with the exception that the layer of mesoblast, which is undoubtedly present as an inner lining to the wall of the vesicle, is omitted. In Fig. 109, A, the commencement of the for mation of the embryo is indicated; the germinal area has become some what depressed, but at its anterior end, (to the right in the figure), is lifted up slightly by a constriction—the head-fold. In Fig. 109, B, this head-fold has deepened, and the head-end of the embryo is now distinctly raised above the yolk-sac, Y, s; at the hinder end of the embryo the ger minal area, however, still preserves, as in the former figure, its primitive connection with the chorion. In Fig. 109, C, the embryo has reached the stage shown in Fig. 108; indeed, Fig. 109, C, is a diagrammatic longitudi nal section through this very embryo. At the hinder end of the embryo the tail-fold has now just commenced; but this does not interfere with the stalk—the allantoic stalk—which still connects directly together the embryo and the horion.