Plate 16

Copyright © 1995
to the
"Frontiers in Bioscience"

Midgastrula stage (Embryo stage 11 1/2)

The bottle cells which start to appea r at the onset of gastrulation move at the mid-gastrula stage inward. This causes formation of the tip of the arachenteron. At this stage. a specific structure is formed named arachenteron. The roof of this structure is formed by the superficial suprablastoporal and involuted epithelial cells. The tip of the arachenteron is made of the bottle cells that moved inward. These cells start to appear at the onset of gastrulation. The axial mesoderm is formed by cells in the dorsal suprablastoporal region which shift dorsally by the convergence of the dorsal regions and the by animalward extensions. The dorsal contains a massive accumulation of cells which have been shifted dorsally from the lateral re-gions by dorsal convergence and animalwards by extension. The axial mesoderm begins to form from these cells. Cell arrangement within the dorsal blastopore lip appears chaotic, an ordered involution around an inner lip cannot be dis-cerned. The vegetal yolk mass is "overrolled" by the advancing dorsal blastopore lip. Spreading mesodermal cells from the deep zone and adja-cent endodermal cells from the vegetal yolk mass are approaching the animal pole. Larger and further advanced mesodermal cells are not easily distinguished from endodermal cells. The three germ layers begin to become discernible. As a first sign of neural induction the cells of the sen-sorial layer of the dorsal ectoderm (prospective neurectoderm) begin to elongate and form a col-umnar epithelium. Contrary to current theories on neural induction, this happens also at the ani-mal pole, which is not in contact with mesoder-mal cells. Bottle cell formation has reached the ventral side, where the number of prospective mesodermal cells migrating in the animal direc-tion is smaller than on the dorsal side. The de-pression at the blastocoel roof is an artifact re-sulting from the embedding procedure.