317a cb dB
BPPPSNN KSBNEM cellsbcFig. 7.4 Gastrulation in different vertebrate species. (a) Gastrulation in the chicken begins with
the circular polonaise movements (arrows) of epiblast cells toward the posterior midline, where
convergence and extension movements initiate formation of the primitive streak anterior to the
Posterior Marginal Zone (PMZ). As the primitive streak extends, the Node forms as an epithelial
thickening at the anterior end of the streak. Presumptive mesodermal and endodermal cells ingress
from the epiblast. (b) In turtles, gastrulation begins with the flattening of the epithelium in the
posterior margin, forming the Primitive Plate. An amphibian-like blastopore forms anterior to the
primitive plate. Presumptive mesoderm and endoderm involute in the blastopore, and lose epithe-
lial polarity (black arrows). Cells ingress from the epiblast after internalization. Mesoderm and
endodermal cells ingress from the Primitive Plate to generate an uncharacterized tissue type. (c)
Gastrulation begins in amphibians with the apical constriction of a small group of cells, called the
bottle cells, in the subequatorial region of the dorsal margin. Constriction of the bottle cells draws
the attached vegetal cells upward into the blastopore, and the bottle cells displace the involuting
presumptive mesoderm inside the embryo and toward the animal pole (white arrows). Involution
occurs all around the margin, but begins later on the lateral and ventral sides. (d) Gastrulation in
teleosts begins when epiboly movements begin to spread the epiblast over the extraembryonic yolk
(black arrows). A group of non-involuting, endocytic marginal cells (NEM) forms at the dorsal
margin. At later stages, these cells become the dorsal forerunner cells. When cells reach the equa-
tor, presumptive mesoderm and endoderm internalizes around the entire margin by a polyingres-
sion mechanism (white arrows), which resembles the wheeling-in movement of involution. In the
embryonic shield, cells also involute from the epiblast by ingression (black). Blue = epiblast,
Red = presumptive mesoderm, Green = presumptive endoderm, grey = extraembryonic. B blasto-
pore, bc bottle cell, KS Koller’s sickle, N node, NEM non-involuting, endocytic marginal cells, PP
primitive plate, PS primitive streak
7 Establishment of the Vertebrate Germ Layers