At the close of segmentation ([fig. 247] A) the egg has a nearly spherical form, and is constituted of a single layer of columnar cells enclosing a small segmentation cavity. The lower pole is slightly thickened, and the egg rotates by means of fine cilia.
An invagination now makes its appearance at the lower pole ([fig. 247] B), and simultaneously there become budded off from the cells undergoing the invagination amœboid cells, which eventually form the muscular system and the connective tissue. These cells very probably have a bilaterally symmetrical origin. This stage represents the gastrula stage which is common to all Echinoderms. The invaginated sack is the archenteron. As it grows larger one side of the embryo becomes flattened, and the other more convex. On the flattened side a fresh invagination arises, the opening of which forms the permanent mouth, the opening of the first invagination remaining as the permanent anus ([fig. 248] A).
Fig. 247. Two stages in the development of Holothuria tubulosa viewed in optical section. (After Selenka.)
A. Blastosphere stage at the close of segmentation. B. Gastrula stage.
mr. micropyle; fl. chorion; s.c. segmentation cavity; bl. blastoderm; ep. epiblast; hy. hypoblast; ms. amœboid cells derived from hypoblast; a.e. archenteron.
These changes give us the means of attaching definite names to the various parts of the embryo. It deserves to be noted in the first place that the embryo has assumed a distinctly bilateral form. There is present a more or less concave surface extending from the mouth to near the anus, which will be spoken of as the ventral surface. The anus is situated at the posterior extremity. The convex surface opposite the ventral surface forms the dorsal surface, which terminates anteriorly in a rounded præ-oral prominence.
It will be noticed in [fig. 248] A that in addition to the primitive anal invagination there is present a vesicle (v.p.). This vesicle is directly formed by a constriction of the primitive archenteron ([fig. 249] Vpv.), and is called by Selenka the vaso-peritoneal vesicle. It gives origin to the epithelioid lining of the body cavity and water-vascular system of the adult[218]. In the parts now developed we have the rudiments of all the adult organs.
The mouth and anal involutions (after the separation of the vaso-peritoneal vesicle) meet and unite, a constriction indicating their point of junction ([fig. 248] B). Eventually the former gives rise to the mouth and œsophagus, and the latter to the remainder of the alimentary canal[219].