_Reproductive Organs._--In the cockroach embryo, before the segmentation of the germ-band has begun, the primitive germ-cells can be recognized at the hinder end of the mesoderm, from whose ordinary cells they can be distinguished by their larger size. At a later stage further germ-cells arise from the epithelium of the coelomic pouches from the second to the seventh abdominal segments, and become surrounded by other mesoderm cells which form the ovarian or testicular tubes and ducts (fig. 19, g). In the male of _Phyllodromia_ the rudiment of a vestigial ovary becomes separated from the developing testis, indicating perhaps an originally hermaphrodite condition. An exceedingly early differentiation of the primitive germ-cells occurs in certain Diptera. E. Metchnikoff observed (1866) in the development of the parthenogenetic eggs produced by the precocious larva of the gall-midge _Cecidomyia_ that a large "polar-cell" appeared at one extremity during the primitive cell-segmentation. This by successive divisions forms a group of four to eight cells, which subsequently pass through the blastoderm, and dividing into two groups become symmetrically arranged and surrounded by the rudiments of the ovarian tubes. E. G. Balbiani and R. Ritter (1890) have since observed a similar early origin for the germ-cells in the midge _Chironomus_ and in the _Aphidae_. The paired oviducts and vasa deferentia are, as we have seen, mesodermal in origin. The median vagina, spermatheca and ejaculatory duct are, on the other hand, formed by ectodermal inpushings. The classical researches of J. A. Palmen (1884) on these ducts have shown that in may-flies and in female earwigs the paired mesodermal ducts open directly to the exterior, while in male earwigs there is a single mesodermal duct, due either to the coalescence of the two or to the suppression of one. In the absence of the external ectodermal ducts usual in winged insects, these two groups resemble therefore the primitive Aptera. The presence of rudiments of the genital ducts of both sexes in the embryo of either sex is interesting and suggestive. The ejaculatory duct which opens on the ninth abdominal sternum in the adult male arises in the tenth abdominal embryonic segment and subsequently moves forward. GROWTH AND METAMORPHOSIS [Illustration: After Marlatt, _Ent. Bull._ 4, n. s. (U.S. Dept. Agr.). FIG. 20.--a, Bed-bug (_Cimex lectularis_, Linn.); newly hatched young from beneath; b, from above; d, egg, magnified; c, foot with claws; e, serrate spine, more highly magnified.] [Illustration: From Mally, _Ent. Bull._ 24 (U.S. Dept. Agr.). FIG. 21.--e, f, Owl moth (_Heliothis armigera_); a, b, egg, highly magnified; c, larva or caterpillar; d, pupa in earthen cell.]