But unfortunately the development has only been fully worked out in one species, and to that species—_P. capensis_—the following description refers. The ova are apparently fertilised in the ovary, and they pass into the oviducts in April and May. In May the brood of the preceding year are born, and the new ova, which have meanwhile undergone cleavage, pass into the uterus. There are ten to twenty ova in each uterus. The segmentation is peculiar, and leads to the formation of a solid gastrula, consisting of a cortex of ectoderm nuclei surrounding a central endodermal mass, which consists of a much-vacuolated tissue with some irregularly-shaped nuclei. The endoderm mass is exposed at one point—the blastopore (gastrula mouth). The central vacuoles of the endoderm now unite and form the enteron of the embryo, and at the same time the embryo elongates into a markedly oval form, and an opacity—the primitive streak—appears at the hind end of the blastopore (Fig. 13, B). This elongation of the embryo is accompanied by an elongation of the blastopore, which soon becomes dumb-bell shaped (Fig. 13, C). At the same time the mesoblastic somites (embryonic segments of mesoderm) have made {21}their appearance in pairs at the hind end, and gradually travel forward on each side of the blastopore to the front end, where the somites of the anterior pair soon meet in front of the blastopore (Fig. 13, D). Meanwhile the narrow middle part of the blastopore has closed by a fusion of its lips, so that the blastopore is represented by two openings, the future mouth and anus. A primitive groove makes its appearance behind the blastopore (Fig. 13, D). At this stage the hind end of the body becomes curved ventrally into a spiral (Fig. 13, E), and at the same time the appendages appear as hollow processes of the body wall, a mesoblastic somite being prolonged into each of them. The first to appear are the antennae, into which the praeoral somites are prolonged. The remainder appear from before backwards in regular order, viz. jaw, oral papillae, legs 1-17. The full number of somites and their appendages is not, however, completed until a later stage. The nervous system is formed as an annular thickening of ectoderm passing in front of the mouth and behind the anus, and lying on each side of the blastopore along the lines of the somites. The praeoral part of this thickening, which gives rise to the cerebral ganglia, becomes pitted inwards on each side (Fig. 13, F, _c.g_). These pits are eventually closed, and form the hollow ventral appendages of the supra-pharyngeal ganglia of the adult (Fig. 9, _d_). The lips are formed as folds of the side wall of the body, extending from the praeoral lobes to just behind the jaw (Fig. 13, F, _L_). They enclose the jaws (_j_) mouth (_M_), and opening of the salivary glands (_o.s_), and so give rise to the buccal cavity. The embryo has now lost its spiral curvature, and becomes completely doubled upon itself, the hind end being