A. The gastrula stage. B. A succeeding stage in which the primitive archenteron is commencing to be divided into three parts, the two lateral of which are destined to form the body cavity. C. A later stage in which the mouth involution (_m_) has become continuous with the alimentary tract, and the blastopore has become closed. _m._ mouth; al. alimentary canal; _ae._ archenteron; _bl.p._ blastopore; _pv._ perivisceral cavity; _sp._ splanchnopleuric mesoblast; _so._ somatopleuric mesoblast; _ge._ generative organs.] At the gastrula stage there is formed a paired mass destined to give rise to the generative organs. It arises as a prominence of six cells, projecting from the hypoblast at the anterior pole of the archenteron, and soon separates itself as a mass, or probably a pair of masses, lying freely in the cavity of the archenteron (fig. 164 A, _ge_). When the folding of the primitive cavity takes place the generative rudiment is situated at the hind end of the median lobe of the archenteron in the position represented in fig. 164 C, _ge_. An elongation of the posterior end of the embryo now takes place, and the embryo becomes coiled up in the egg, and when eventually hatched sufficiently resembles the adult to be recognisable as a young Sagitta. Before hatching takes place various important changes become manifest. The blastopore disappears after being carried to the ventral surface. The middle section of the trilobed region of the archenteron becomes separated from the unpaired posterior part, and forms a tube, blind behind, but opening in front by the mouth (fig. 165 A, _al_). It constitutes the permanent alimentary tract, and is formed of a pharyngeal epiblastic invagination, and a posterior hypoblastic section derived from the primitive archenteron. The anus is apparently not formed till comparatively late. After the isolation of the alimentary tract the remainder of the archenteron is formed of two cavities in front, which open freely into a single cavity behind (fig. 165 A). _The whole of it constitutes the body cavity and its walls the mesoblast._ The anterior paired part becomes partitioned off into a head section and a trunk section (fig. 165 A and B). The former constitutes a pair of distinct cavities (_c.pv_) in the head, and the latter two cavities opening freely into the unpaired portion behind. At the junction of the paired cavities with the unpaired cavity are situated the generative organs (_ge_). The inner wall of each of the paired cavities forms the splanchnopleuric mesoblast, and the outer wall of the whole the somatic mesoblast. The inner walls of the posterior cavities unite above and below the alimentary tract, and form the dorsal and ventral mesenteries, which divide the body cavity into two compartments in the adult. Before the hatching of the embryo takes place this mesentery is continued backwards so as to divide the