The hypoblast is formed either by a definite invagination or by the immigration of isolated cells, known as vitellophags, which wander through the yolk and later become associated into a definite mesenteron, or by some combination of these two methods.
Similarly an anal aperture is formed in connexion with a special invagination which meets the hinder part of the met-enteron, and is termed the proctodaeum.
The orifice of invagination forms a pore which may be closed up or may form a protruding duct or funnel.
Hence the cavity of the air-sack is equivalent to a sub-umbral cavity in which no manubrium is formed, and the pore or orifice of invagination would represent the margin of the umbrella.
This bilobed sac becomes entirely the liver in the adult; the intestine and stomach are formed from the pedicle of invagination, whilst the pharynx, oesophagus and crop form from the stomodaeal invagination ph.
After the Actinotrocha has led a pelagic, life for some time it develops a large ventral invagination of its body-wall (fig.
The proboscis is an invagination from the epiblast; the proboscidian sheath appears in the mesoblast, but is perhaps originally derived from the hypoblast.
Io, B) in the gastrula stage, and the orifice of invagination or blastopore, which persists, is situated at the hinder pole.
There is, strictly speaking, no invagination (emboly), but an overgrowth (epiboly) of the smaller cells to enclose the larger.
The shell-gland, sk, is later stage, in which a flattened out, and a delicate second invagination has beshell, s, appears on its sur gun - namely, that of the face.