Arch-enteron or cavity lined by the enteric cell-layer or endo derm.
The invaginated cells (derived from the division of the four big cells) form the endoderm or arch-enteron; the outer cells are the ectoderm.
Ing gs, the bilobed arch sh, The primitive shell-sac or enteron or lateral vesicles shell-gland ° of invaginated endoderm, pi, The rectal peduncle or whichwill develop into liver.
The body-cavity and the muscular, fibrous and vascular tissues are traced partly to two symmetrically disposed " mesoblasts," which bud off from the invaginated arch-enteron, partly to cells derived from the ectoderm, which at a very early stage is connected by long processes with the invaginated endoderm.
The mass of the arch-enteron or invaginated endodermal sac has taken on a bilobed form, and its cells are swollen (gs and tge).