[Illustration: FIG. 67.—_Ascaris lumbricoides_ Cloq. ♀, natural size, cut open along the median dorsal line to show the internal organs. _a_, The muscular oesophagus; _b_, the intestine; _c_, the ovary; _d_, the uterus; _e_, the vagina; _f_, the external opening; _h_, the excretory canals; _i_, their opening.] EMBRYOLOGY.—The eggs of many of the parasitic forms require a considerable degree of warmth to develop. Those of _Ascaris lumbricoides_ require a temperature of 20° C., those of _Trichocephalus_ 22.5° C., and those of _Oxyuris vermicularis_, 40° C. The latter develop in a few hours, the eggs of _Dochmius_ in a few days, whilst those of _A. {136}lumbricoides_ take weeks or even months, and the young of _Trichocephalus_ seldom develop within a year.[168] The ova only develop in a damp atmosphere, and they can be arrested at almost any stage, and for considerable periods, by desiccation. Our knowledge of the processes by which the fertilised egg-cell develops into the larva is very imperfect. As a rule the segmentation is complete and equal; it results in the formation of a blastula, which may take the form of a hollow sphere of cells—_A. megalocephala_—or the cavity may be reduced, and the blastula may consist of a double-layered plate, as in _Cucullanus_.[169] The distinction into cells which will form the three embryonic layers, the ectoderm, mesoderm, and endoderm, is very early evident,—in the eight-cell stage. By the growth of one side of the blastula and the tucking in of the other the blastula becomes converted into a gastrula, which is a two-layered stage with a cavity opening to the exterior by a pore termed the blastopore. In Nematodes the blastopore is elongated and slit-like; it either forms the mouth (_Cucullanus_) or closes from behind forwards, the mouth ultimately arising at the point where the blastopore finally closed (_Rhabdonema nigrovenosum_). The mesodermal cells lie between the ectoderm and the endoderm; they ultimately develop into the muscles of the body-wall, the lateral excretory canals, and the reproductive organs; the last-named two systems arise each[170] from a single cell. The nervous system arises from the ectoderm, which also forms the sub-cuticle, and is turned in slightly at the mouth and anus; the remainder of the alimentary canal develops from the endoderm. The post-embryonic development, which is very variable, and in many cases very extraordinary, will be dealt with under the several families.