Micromere descendants at the blastula stage are involved in normal archenteron
formation in sea urchin embryos.
By this point, gastrulation has taken place and the archenteron is well developed.
In the polychaete Eunice kobiensis, the anus also forms in the region in which the archenteron opens, and the stomodeum forms as a separate, secondary invagination (Akesson, 1967).
A true gastrula with sealed epihelia and an archenteron
is seen in all eumetazoans, both in the larval and adult organization of enidarians and ctenophores (and acoelomorphs) and as an ontogenetic stage in eubilaterians.
The archenteron of both types differentiated into intestine, stomach, and esophagus.
scoparius, while others had a nonfunctional gut lacking a connection between the anterior tip of the archenteron and the stomodeum.
During secondary invagination, a population of secondary mesenchyme cells (SMCs) connect the archenteron
tip to the inner surface of the apical plate and exert the force to pull up the archenteron
(Dan and Okazaki, 1956; Hardin, 1988).
Mesenchymal cells are present in the blastocoel (of the arms), but are associated with the skeletal rods and do not seem to contribute to the formation of the archenteron
or the axohydrocoels.
Gastrulation in echinoderms involves two simultaneous but relatively independent processes, invagination to form the archenteron
and migration of the primary mesenchyme cells.
exigua are (1) the absence of the bipinnaria larva and ciliated bands; (2) the early closure of the blastopore; and (3) the failure of the archenteron
to connect with a stomodeum.
As a result of these vegetal cell movements, an archenteron
resembling that of B.
At 24 h postfertilization, at which time control embryos had formed gastrulae, the embryos treated with acidified seawater alone were morphologically abnormal, many with distorted shapes or abnormal archenterons
, and some embryos had multiple archenterons