The slender terminal thread serves to attach or suspend each egg-tube near the dorsal vessel (not directly to the heart, as formerly supposed), becoming lost in the fat-body.
Fig. 473.—Ovarian tube of P. orientalis: A, section near the end; tf, base of terminal filament. B, section lower down; ec, egg-cells in egg-chamber.—After Brandt.
The terminal chamber contains undifferentiated cell elements, supposed to be the remains of the ovarian rudiments. From these arise (either in the embryo or larva) first, the follicle epithelium of the ovarian tubes; and, second, the material for the formation of the new eggs, and nutritive cells. “In the terminal chamber these cell-elements remain undifferentiated, excepting when required for the removal of the follicle epithelium, eggs, and nutritive cells in the adult insect.” (Lang.) This portion of the ovariole is called the germarium. In Blatta it is filled with protoplasm in which numerous small nuclei are imbedded. (Wheeler.) The chambered main division of the egg-tube contains the ripening eggs, one in each compartment, the tube appearing like a string of beads.
The egg-tubes are of two types: (1) those without, and (2) those with nutritive cells, the first kind being the simplest, and occurring in the Synaptera (except Campodea) and in Orthoptera. As an example may be cited that of the cockroach (Fig. 473), where in each tube there is a simple continuous row of eggs from the terminal chamber to the oviduct. The tube being constricted between these consecutive eggs, gives it a beaded appearance.
In the cockroach (Periplaneta orientalis) each egg-tube has a beaded appearance. Its wall consists of a transparent elastic membrane, lined by epithelium, with an external peritoneal layer of connective tissue. The terminal filament (tf) is filled with a clear protoplasm, with a few nuclei. In the terminal chamber (tc) are large nucleated cells, with separate nuclei, both entangled in a network of protoplasm. In the third, or egg-chamber (ec), are about twenty ripening eggs, arranged in a single row. “Between and around the eggs the nuclei gradually arrange themselves into one-layered follicles, which are attached, not to the wall of the tubes, but to the eggs, and travel downwards with them. As the eggs descend, the yolk which they contain increases rapidly, and the germinal vesicle and spot (nucleus and nucleolus), which were at first plain, disappear. A vitelline membrane is secreted by the inner surface and a chitinous chorion by the outer surface of the egg-follicle.
“The lowest egg in an ovarian tube is nearly or altogether of the full size; it is of elongate-oval figure, and slightly curved, the convexity being turned towards the uterus. It is filled with a clear albuminous fluid, which mainly consists of yolk. The chorion now forms a transparent yellowish capsule, which, under the microscope, appears to be divided up into very many polygonal areas, defined by rows of fine dots. These areas probably correspond to as many follicular cells.” (Brandt, from Miall and Denny.)
In the second type, i.e. those egg-tubes with nutritive cells, there are two kinds. In the first the egg-chambers and yolk- or nutritive chambers alternate, each of the latter containing one or more nutritive cells, which serve for the nourishment of the ripening egg contained in the neighboring chamber. “The egg- and yolk-chambers may be distinctly separated externally by constrictions (Hymenoptera and many Coleoptera), or one nutritive and one egg-chamber may lie in each section of the ovarian tube, which is externally visible as a swelling (Lepidoptera, Diptera).”
In the second kind with nutritive cells, the actual tube consists (Fig. 474, C) of ovarian chambers only; the nutritive cells here remain massed together in the large terminal chamber. The single egg in the tube is united with the terminal chamber by connective strands (d. s.), which convey the nutritive material to the eggs. (Lang.)
Egg-cells, nutritive cells, and the cells of the follicle-epithelium (epithelium of the chambers of the ovarian tubes) are, says Lang, according to their origin, similar elements, like the egg and yolk-cells of the flat worms (Platodes); division of labor leads to their later differentiation. Only a few of the numerous egg-germs develop into eggs, the rest serving as envelopes and as food for these few.