The ova originate from certain cells (germinal cells) in the peritoneal investment of the ventral vessel, overlying the nervous cord. These cells, which are well marked off from the surrounding flattened peritoneal elements, increase in number by division, and form small masses surrounded by a follicle of peritoneal cells, and attached by a stalk to the peritoneum. The central cell of each mass grows larger than the rest, which arrange themselves in a columnar fashion round it; it is not, however, destined to become the ovum. On the contrary certain of the other cells adjoining the stalk grow larger, and finally one of these becomes distinguished as the ovum by its greater size and the character of its nucleus. The remainder of the larger cells become of the same size as their neighbours. The ovum now becomes more or less separate from the mass of germinal cells, rapidly grows in size, and soon forms the most considerable constituent of the follicle ([fig. 16], ov). The remaining germinal cells are quite passive, and though, with the exception of the central cell, they do not appear to atrophy, they soon constitute a relatively small prominence on the surface of the ovum. By the rupture of the stalk the whole follicle becomes eventually detached, and the further development of the ovum takes place in the body cavity. A vitelline membrane is formed, and eventually the ovum is taken into the oviduct (segmental organ). At this time or slightly before, the follicle cells together with the germinal mass, which throughout exhibits no signs of atrophy, become thrown off, and the ovum is left invested in its vitelline membrane.
Nematoda.
(44) Ed. Claparède. De la formation et de la fécondation des œufs chez les Vers Nématodes. Genève, 1859.
(45) R. Leuckart. Die menschlichen Parasiten.
(46) H. Munk. “Ueb. Ei- u. Samenbildung u. Befruchtung b. d. Nematoden.” Zeit. f. wiss. Zool., Vol. IX. 1858.
(47) H. Nelson. “On the reproduction of Ascaris mystax, etc.” Phil. Trans. 1852.
(48) A. Schneider. Monographie d. Nematoden. Berlin, 1866.
The female organs consist as a rule of two cæcal tubes which unite before opening to the exterior. Each of these is divided into a vagina, uterus, oviduct, and ovary. The ovary constitutes the blind end of the tube, and is formed of a common protoplasmic column, holding a number of nuclei in suspension. The protoplasm becomes cleft around the nuclei in the uppermost part of the tube; the circumscription of the ova proceeds, however, very gradually, and since it commences at the periphery of the column the ova remain attached by stalks to a central axis with one end free. In this way there is formed a rod-like structure known as the rachis, which consists of a central axis with a series of half circumscribed ova radiately arranged round it. In the lowest part of the ovary the ova become completely isolated and form separate cells.
The protoplasm of the ova, which is clear in the terminal division of the ovary, becomes in most forms filled lower down with yolk-spherules secreted in the body of the ova. These commence to appear at the uppermost extremity of the rachis.
In some instances, e.g. Cucullanus elegans, yolk-spherules are not formed. In the Oxyuridæ the ova are directly segmented off from the terminal syncytium of protoplasm without the intervention of a rachis; and are therefore formed in the same way as amongst Trematodes, etc.
The origin of the membrane around the ova of the Nematoda has been much disputed.
At the time when the ovum is detached from the rachis no membrane is present, but it nevertheless appears from Schneider’s observations that the region at which it is detached is softer than other parts, so that a kind of micropyle is here formed which disappears after impregnation. A delicate vitelline membrane then appears, around which there is subsequently established an egg-shell, which is usually stated to be formed as a secretion of the walls of the uterus; but Schneider and Leuckart have given strong grounds for believing that it is really a further differentiation of the vitelline membrane due to the activity of the protoplasm of the ovum. The originally single membrane becomes as it thickens split into two layers. The outer of these forms the true egg-shell, and the fertilization of the ovum appears to be a necessary prelude to its production. Round the egg-shell the walls of the uterus often secrete a special albuminous covering.
The egg-shell exhibits in many cases peculiar sculpturings as well as terminal prolongations.
Insecta.