From the agreement of the position of the sexual openings in Phyllodromia with the conditions observed in the Ephemeridæ, with which the Perlidæ also agree, we conclude that in the entire group of insects an opening between the 7th and 8th abdominal segments is the primitive condition, and that only by a secondary shifting has a more posterior position of the opening (in many forms) been brought about. In this category we must certainly include the Thysanura, in which the sexual opening is single and situated between the 8th and 9th abdominal segments.
Development of the male germinal glands.—These rudiments arise in exactly the same manner as those of the female. Sexual differentiation takes place in the later embryonic stages. We then notice that in the male four masses of genital cells become surrounded by epithelial cells. These masses, which form the germs of the four testicular follicles of Phyllodromia, stand in intimate union with the rudiment of the vas deferens, and in the later stages move in connection with the latter, away from and behind the original genital rudiment. There remains, then, with the terminal thread-plate a remnant of the genital rudiment, which, according to Heymons, forms the female part of the original hermaphroditic genital rudiment, and in special cases may develop even into rudimentary egg-tubes and eggs. The rudimentary organ arising out of this genital rudiment may also be demonstrated in the adult male of Phyllodromia.
In the female the oviduct arises directly out of the originally established efferent passage. In the male, on the contrary, it is not, along its whole length, transformed into the vas deferens, but its distal terminal portion degenerates and is replaced by a newly formed terminal portion of the vas deferens, which then unites with the ectodermal ductus ejaculatorius. (Korschelt and Heider.)
On reviewing the facts as to the origin of the sexual organs, as in Phyllodromia,[[84]] as just described, it will be seen that they afford proof that in the derivation of the genital cells from the epithelial cells of the cœlom-sacs, there is a direct agreement with the annelids. In the later development of the paired genital glands, and of an efferent passage standing in direct union with the glands themselves, there is a certain agreement with the conditions in Peripatus. In the first place, the dorsal position of the genital glands is the same in the two groups. On the other hand, the genital glands of Peripatus, according to Sedgwick, are formed by direct fusion of the successive cœlom-sacs (and a similar point of view has been taken by Heathcote for the myriopods), hence it results that in Peripatus the genital cavities arise out of the cœlom-cavities. In the insects, on the other hand, the genital rudiment lies, to be sure, in the wall of the cœlom-sac, but the genital cavity (lumen of the oviducts) in them arises separately from the cœlom-sacs, while the cœlom-cavities finally become a small part of the definite body-cavity. We must consider the conditions in Peripatus and the myriopods as the more primitive, directly pointing to the annelids; on the other hand, those of the insects as derived and secondary.
If we attempt to homologize the sexual efferent passages of insects with those of Peripatus, we are compelled to refer them to a modified pair of nephridia, and the origin of the latter (Peripatus) from the mesoderm agrees with that of insects. In general, however, in the development of the sexual outlets of insects, there are no characters which can be regarded as favorable to such a view. We must here accept the fact that the mode of development is secondary.
Mention should be specially made of the fact we owe to Heymons, that in the genital rudiment of Phyllodromia the genital cells and epithelial cells can be distinguished from each other from the very beginning. This fact does not favor the generally accepted view that the follicle-cells and egg-cells arise through a later differentiation from one and the same kind of cell. From their first origin, indeed, in Phyllodromia, both kinds of cells may be referred to the same source.
The mode of origin of the genital rudiments in Diptera and Aphides deserve special mention. In these groups the sexual germs are present in very early stages of life. This certainly in part is the result of the parthenogenetic and pædogenetic mode of reproduction in the two groups, which leads to an early differentiation of the sexual germs.
In the Diptera the first germs of the genital glands are represented by the polar cells (Fig. 551, pz). In the asexual developing eggs of the oviparous Cecidomyia larva, before the formation of the blastoderm, there separates from the hinder pole (D) a rather large cell rich in granules, which soon divides into two and afterwards four polar cells. After the completion of the blastoderm these polar cells then pass in among the blastoderm cells (G) and into the interior of the embryo, where they are in later stages symmetrically arranged in two groups, and, enveloped by the cells of surrounding tissues, transformed into the genital rudiments. (Metschnikoff.)
In Chironomus (Fig. 552, p), according to Balbiani, two polar cells almost simultaneously separate from the hinder pole of the egg, which, by division, form a group of four and eight cells. Exactly as in the case in Cecidomyia, these cells are taken within the embryo, where they lie divided into two groups on each side of the proctodæum. In all the young, freshly hatched larvæ; these two spindle-shaped groups, whose cells soon increase in number, may be seen situated dorsally on the side of the heart, enveloped by a clear cellular membrane which ends before and behind in a ligament-like terminal thread. The anterior terminal thread is the rudiment of the so-called Müller’s thread. The thread at the posterior end is the rudiment of the paired efferent passage of the genital glands. Through a division of the cells lying in the interior of the rudiments of the ovaries, there results the formation of a rosette-shaped group of cells which corresponds to the contents of an ovarian tube. With this view of Balbiani the later observations of Ritter agree.
As in the Diptera, so in the Aphides, the first germs of the genital organs are differentiated very early in life. In the early stage in which through an invagination from the hinder pole of the egg the first rudiment of the amnion-cavity is formed, a group of cells becomes separated from the wall of this invagination before the formation of the lower layer, which at this time lies as an unpaired roundish mass within the embryo. This group of cells, according to Balbiani and Witlaczil, has arisen by division of a single cell. Afterwards it becomes horseshoe-shaped and divides into a number of roundish masses of cells, which are arranged in similar numbers on each side of the median plane of the body, and form the rudiments of the terminal fan (Endfächer). They are covered by an epithelial envelope which passes anteriorly into the terminal threads, posteriorly into the efferent passage. The origin of this epithelial case is unknown. The efferent passages of the separate ovarian tubes are united into a common oviduct, and this fuses with an unpaired ectodermal invagination lying under the hind intestine from which the accessory sexual organs are formed. (Korschelt and Heider from Metschnikoff, Witlaczil, Will.)