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]).
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.
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 amœboid 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.