Gephyrea.--Bonellia. It arises (fig. 206) as an infolding of the epiblastic lips of the blastopore. Nematelminthes.--Cucullanus. It grows backwards from the hypoblast cells at the persistent oral opening of the blastopore. Tracheata.--Insecta. It grows inwards from the lips of the germinal groove (fig. 207), which probably represent the remains of a blastopore. Part of the mesoblast is probably also derived from the yolk-cells. A similar though more modified development of the mesoblast occurs in the Araneina (fig. 214). Crustacea.--Decapoda. It partly grows in from the hypoblastic lips of the blastopore, and is partly derived from the yolk-cells (fig. 208). [FIG. 209. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A. and C. after Bütschli, and B. after Kowalevsky.) The three embryos are represented in the same positions. A. Represents the gastrula stage. B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three. C. Represents 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._ splanchnic mesoblast; _so._ somatic mesoblast; _ge._ generative organs.] 2. The mesoblast is developed from the walls of hollow outgrowths of the archenteron, the cavities of which become the body cavity. Brachiopoda.--The walls of a pair of outgrowths form the whole of the mesoblast. Chætognatha.--The mesoblast arises in the same manner as in the Brachiopoda (fig. 209). Echinodermata.--The lining of the peritoneal cavity is developed from the walls of outgrowths of the archenteron, but the greater part of the mesoblast is derived from the amoeboid cells budded off from the walls of the archenteron (fig. 210). [FIG. 210. LONGITUDINAL SECTION THROUGH AN EMBRYO OF CUCUMARIA DOLIOLUM AT THE END OF THE FOURTH DAY. _Vpv._ vaso-peritoneal vesicle; _ME._ mesenteron; _Blp._, _Ptd._ blastopore, proctodæum.] Enteropneusta (Balanoglossus).--The body cavity is derived from two pairs of alimentary diverticula, the walls of which give rise to the greater part of the mesoblast. Chordata.--Paired archenteric outgrowths give rise to the whole mesoblast in Amphioxus (fig. 211), and the mode of formation of the mesoblast in other Chordata is probably secondarily derived from this. 3. The cells which will form the mesoblast become marked out very early, and cannot be regarded as definitely springing from either of the primary layers. Turbellaria.--Leptoplana (fig. 212), Planaria polychroa (?). Chætopoda.--Lumbricus, &c. Discophora. It is very possible that the cases quoted under this head ought more properly to belong to group 1. 4. The mesoblast cells are split off from the epiblast. Nemertea.--Larva of Desor. The mesoblast is stated to be split off from the four invaginated discs.