The cœlom-sacs in the farther course of their development, through the retrograde development of the parts extending into the appendages, through the development of the fat-bodies and through the delamination of the somatic and the splanchnic mesoderm layer, become greatly diminished in size. Finally, there remains left of them only a rather small cavity (c), which is bordered on the side by the rudiment of the pericardial septum (ps) and within by the terminal thread-plate (ef). The dorsally situated point where these two lamellæ pass into each other seems to stand in intimate connection with the cells of the rudiment of the heart (h). The cord-like genital rudiment hangs from the terminal thread-plate as from a mesentery (Fig. 549, gz).
Fig. 550.—Longitudinal section through the female genital rudiments of P. germanica. A, with beginning, B, with farther advanced growth of the ovarian tubes: cz, rudiment of the genital efferent passage; ef, terminal threads; ep, nucleus of the epithelial cells; gz, genital cells.—After Heymons, from Korschelt and Heider.
Together with the growth of the primitive band around the yolk, and the formation of the back, the paired rudiments of the heart gradually extend to the neighborhood of the dorsal median line, followed by the genital rudiments which are connected with them by the terminal thread-plates. The genital rudiments advance thus to the dorsal side of the developing mid-intestine (Fig. 545, gz).
The terminal thread-plate (ef) is at first a simple epithelial plate. Soon, however, follows an arrangement of its cells whereby they appear to be arranged in vertical rows, each one of which corresponds to a developing ovarian tube. In this way the terminal thread-plate separates into the separate terminal threads of the ovarian tubes (Fig. 550, ef). In this process of division the uppermost dorsal edge of the terminal thread-plate takes no part. From it afterwards grows a thread which extends anteriorly, which becomes the common terminal thread of all the ovarian tubes, the so-called Müller’s thread. This is originally united with the pericardial septum, but seems in later stages to have no longer an intimate connection with it.
The formation of the single ovarian tubes, which in Phyllodromia number about 20, is accomplished by the extension of indentations from the dorsal side towards the ventral side of the ovarian rudiment (Fig. 550). At the same time the epithelial cells (ep), which were originally situated in part between the genital cells, become arranged in the form of an epithelium on the surface of the ovarian tubes, which soon forms on its outer surface a structureless cuticular tunica propria. The outer peritoneal membrane of the ovary becomes formed of the cells of the surrounding tissue of the fat-body.
The genital rudiment originally extends, as already stated, from the 2d to the 7th abdominal segment. In the last, however, the genital cells at first occur only sparingly, and afterwards completely disappear, so that here the genital cord appears composed of epithelial cells only. This part is the rudiment of the oviduct proper, and forms a direct continuation of the above-mentioned cell-cord which is situated ventralward from the genital cells, from which, as we have seen, the proximal cup-shaped section of the oviduct is formed. The hinder section of the oviduct turns down ventrally in order to unite at the boundary between the 7th and 8th abdominal segments with the hypodermis. The rudiment of the oviduct originally forms a solid strand of cells. Afterwards a cavity is formed by the separation of the cells.
In later stages there is a considerable shortening of the genital rudiment, so that it occupies a smaller number of abdominal segments than at first. At the same time the single ovarian tubes pass out of their originally vertical position into one more horizontal.
The paired connections of the rudiments of the oviducts with the hypodermis of the intersegmental furrow between the 7th and 8th abdominal segments reminds us of the conditions in the Ephemeridæ. This is the primitive condition in insects. In the female of Phyllodromia there is developed during larval life, from an ectodermal invagination, an unpaired terminal section of the genital passage, which becomes the genital pouch in which the egg-case (oötheca) is held. This genital pouch is formed, as Haase has already proved, by the withdrawal of the chitinous ventral plate of the 8th and 9th abdominal segment by invagination into the interior of the body.
The development of the efferent passages has been investigated by Nusbaum in the cockroach (Periplaneta) and in the Pediculina. He found that only the vasa deferentia and the oviducts arise from the hinder cord of the germs of the sexual glands, that is, out of the mesodermal rudiments, while the other parts of the sexual efferent apparatus (uterus, vagina, receptaculum seminis, ejaculatory duct, penis, and all the accessory glands) develop from the integumental epithelium and are of ectodermal origin. In fact, the unpaired parts (uterus, penis, receptaculum seminis, unpaired glands) have developed from paired rudiments, being outgrowths of the hypodermis. The hinder portions of the rudiments of the sexual glands approach these hypodermal growths and fuse with them. Through a median fusion of the paired hypodermal growths arise the germs of the unpaired organs. These observations are in complete agreement with the results at which Palmén arrived by anatomical investigation (see p. 492).