OVULATION.
In the beginning of embryological development, no- trac-- e of i--t can be found; it is only in the fifteenth or sixteenth week after the formation of the so-called primitive kidney or the Wolffian body, that, on the poste rior wall of the abdominal cavity, and on the side of this organ and in connection with it, two white stripes are seen, which.are without doubt to be regarded as a growth of the peritoneal epithelium. Through rapid growth of this germinal epithelium cell-aggregation, this is more sharply defined from the neighboring structures, and appears early in the abdomi nal cavity as an organ supplied with a mesentery, while at the same time the basic membrane also grows and is supplied with vessels.
Until the end of the second month there is in this organ, now called the germinal gland, no sexual difference noticeable; the embryo is still sexless. At this time there first begins that differentiation which makes itself known by the change in the external form, the position and parti cularly in the internal structure, and which eventually leads in females to the development of the ovaries, and in males to the testicles. The trans formation of the germinal gland into the ovary occurs by an intergrowth of the two tissue elements, the germinal epithelium and the basic mem brane. This process is completed in the following way. The germinal epithelium of the surface sends solid, branching processes into the inte rior of that layer, from which organs are developed, where they taike on an appearance similar to the glandular tubes (Pfliiger's tubes). At the satire time connective tissue and cellular trabeculte penetrate from the hilum in processes into the germinal epithelium, which has also become adherent to the centre. Thereby separating walls are formed not only between the trunks and branches of the glandular tubes, but the latter are divided or shut off into single divisions of different sizes by other lateral growths of connective tissue, so that glandular tubes connecting with the surface no longer exist, but the epithelia which join the tubes are pressed into roundish balls, and are surrounded by connective tissue. In this way, then, the two histologically different parts of which the ova ries consist are formed: first, the connective tissue basic membrane, the stroma which forms the principal substance of the organ, and which sends out processes toward the surface, which carry the blood-vessels and nerves, and which contain the smooth muscular tissue; second, the glandular portion, which principally forms the cortical substance, and which con tains the branches of the glandular tubes, the cell-aggregations and later follicles, which are entirely surrounded by the connective-tissue stroma.
But a further important change in the germinal cell occurs before this process of growth. In the superficial layers as well as in those deep epithelial projections, single cells enlarge and assume a spherical form, so that they are more or less sharply defined from the neighboring cells; these are the so-called primitive ova, frOm which the true ovules develop. Now, for the first time, the germinal organ assumes the character of a female sexual organ. These ova may now be imprisoned in the cell aggregations above-mentioned by the occlusion of the glandular tubes, or one cell of these cell-aggregations may be converted into an ovum.
In these cell-aggregations, after their separation, a certain arrangement takes place, by which those cells which have not lost their epithelial character are placed against the wall formed from the stroma, and in clude between them those cells which have become changed into ova. These formations which exist in. a disproportionately considerable number, and which furnish the rudinients of the later Graafian vesicles, are called primary follicles. Only after the termination of the embryonal stage, during which the organ has sunk deeper, even if not completely into the true pelvis, and has assumed an elongated, tongue-like flattened form, do further changes occur in this principal portion of the ovary, in that the true, or so-called Graafian follicles develop from these cell-aggrega tions. Under the influence of those blood-vessels which pass through the stroma, and which surround the single cell-aggregations, a transuda tion takes place between the epithelial cells. By this the space, until now occupied only by the cells, is enlarged, but the epithelial cells, which are greatly increased in number, are, with the ovum, pressed against the side of the cavity. The clear liquid which now occupies the central por tion of the vesicle forms the greater portion of the contents.