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.
Fig. 211. Sections of an Amphioxus embryo at three stages. (After Kowalevsky.)
A. Section at gastrula stage.
B. Section of a somewhat older embryo.
C. Section through the anterior part of still older embryo.
np. neural plate; nc. neural canal; mes. archenteron in A, and mesenteron in B and C; ch. notochord; so. mesoblastic somite.
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 (?).
Fig. 212. Sections through the ovum of Leptoplana tremellaris in three stages of development. (After Hallez.)
ep. epiblast; m. mesoblast; hy. yolk-cells (hypoblast); bl. blastopore.