Fig. 163. Diagrammatic longitudinal section of an ovum of a Guinea-pig and the adjacent uterine walls at an advanced stage of pregnancy. (After Bischoff.)
yk. inverted yolk-sack (umbilical vesicle) formed of an external hypoblastic layer (shaded) and an internal vascular layer (black). At the end of this layer is placed the sinus terminalis; all. allantois; pl. placenta.
The external shaded parts are the uterine walls.
The Human Embryo. Our knowledge as to the early development of the human embryo is in an unsatisfactory state. The positive facts we know are comparatively few, and it is not possible to construct from them a history of the development which is capable of satisfactory comparison with that in other forms, unless all the early embryos known are to be regarded as abnormal. The most remarkable feature in the development, which was first clearly brought to light by Allen Thomson in 1839, is the very early appearance of branched villi. In the last few years several ova, even younger than those described by Allen Thomson, have been met with, which exhibit this peculiarity.
The best-preserved of these ova is one described by Reichert (No. [237]). This ovum, though probably not more than thirteen days old, was completely enclosed by a decidua reflexa. It had ([fig. 164] A and B) a flattened oval form, measuring in its two diameters 5.5 mm. and 3.5 mm. The edge was covered with branched villi, while in the centre of each of the flattened surfaces there was a spot free from villi. On the surface adjoining the uterine wall was a darker area (e) formed of two layers of cells, which is interpreted by Reichert as the embryonic area, while the membrane forming the remainder of the ovum, including the branched villi, was stated by Reichert to be composed of a single row of epithelial cells.
Whether or no Reichert is correct in identifying his darker spot as the embryonic area, it is fairly certain from the later observations of Beigel and Löwe (No. [228]), Ahlfeld (No. [227]), and Kollmann (No. [234]) on ova nearly as young as that of Reichert, that the wall of very young ova has a more complicated structure than Reichert is willing to admit. These authors do not however agree amongst themselves, but from Kollmann’s description, which appears to me the most satisfactory, it is probable that it is composed of an outer epithelial layer, and an inner layer of connective tissue, and that the connective tissue extends at a very early period into the villi; so that the latter are not hollow, as Reichert supposed them to be.
Fig. 164. The human ova during early stages of development. (From Quain’s Anatomy.)
A. and B. Front and side view of an ovum figured by Reichert, supposed to be about thirteen days. e. embryonic area.
C. An ovum of about four or five weeks shewing the general structure of the ovum before the formation of the placenta. Part of the wall of the ovum is removed to shew the embryo in situ. (After Allen Thomson.)
The villi, which at first leave the flattened poles free, seem soon to extend first over one of the flat sides, and finally over the whole ovum ([fig. 164] C).