Stage II ended when the blastopore
was closed and the optic cups distinguished head and tail regions of the embryo.
fuscus appear similar to those of Crepidula fornicata undergoing gastrulation by epiboly, we are uncertain of the origin or consequence of the cellular extensions covering the blastopore
on the ventral surface of the late gastrula stage and cannot entirely rule out the combination of epiboly with other known methods of gastrulation (Verdonk and Biggelaar, 1983).
As mentioned above, the embryonic epidermal epithelium forms early in Xenopus development, and the current flow generated through the embryo by the electrical potential across the epithelium exits the blastopore
(Robinson and Stump, 1984; Hotary and Robinson, 1994).
One week after blastopore
closure (day 22), the tail region began lifting away from the yolk, and within two days, the length of the embryo was 50% of the circumference of the yolk.
At 12 h after spawning, embryos were roughly spherical, with a blastopore
Our conclusion, based on the location of the chaetal sac, the folding of the archenteron, and the relative locations of the stomodeum, anus, and chaetal sac in the earliest swimming larval stage, is that the blastopore
becomes the opening immediately adjacent to the chaetal sac in the larva and thus forms the anus.
Haeckel (1874) suggested that the eumetazoan ancestor was a gastraea with ectoderm, endoderm, and archenteron with blastopore
, essentially having the same structure as that of larval and adult cnidarians and of ontogenetic stages of many bilaterians.
Development of hoplonemerteans, superficially similar to that of palaeonemerteans, is fundamentally different in the fate of larval epidermal cells and blastopore
, and in other morphogenetic events.
Early gastrulae show an enlarged blastopore
whose aperture progressively reduces in older gastrulae (compare Fig.
developed at the middle of the dark cytoplasm, about 24 h post-fertilization, and its rim was marked by dark pigment (Fig.
1H, 2C), with the blastomeres aggregated, at first, around the blastopore
In both species, spiral cleavage and gastrulation by epiboly result in a very slightly flattened gastrula with a central blastopore