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platelets settling in the large prospective endodermal cells at the vegetal pole (Fig.
7.3a) (Karasaki 1963 ). Amphibian embryos undergo holoblastic cleavage, similar to
that observed in placental mammals. At the blastula stage, Na+, K+-ATPase pumps
expressed in the cells at the animal pole are activated and control the influx of fluid
to form a blastocoel underneath the prospective ectoderm (Gosner 1960 ; Morrill
et al. 1974 ).
Gastrulation in Xenopus begins when a small group of presumptive endodermal
cells, called the bottle cells, form in the sub-equatorial zone of the dorsal margin.
These cells undergo a dramatic apical constriction, which generates the dorsal lip of
the blastopore (Fig. 7.4c) (Hardin and Keller 1988 ). Invagination of the bottle cells
internalizes the mesoderm and endoderm in two ways. First, the constriction of the
apical surface results in an elongation of the basolateral surface, which displaces the
presumptive mesoderm internally. Secondly, the vegetal cells attached to the bottle
cells are drawn directly inwards. Presumptive mesoderm internalizes through the
dorsal lip of the blastopore by the process of involution (Fig. 7.4c). According to
John P. Trinkaus (1916–2003), “Involution is the flowing of a sheet of cells over the
edge of an inpocketing, where invagination has occurred, such as the blastopore dur-
ing amphibian gastrulation.” (p. 11) (Trinkaus 1984 ). Alexander Goette (1840–
1922) first described this phenomenon in 1869, in embryos of the European aquatic
toad, Bombinator igneus (Goette 1869 ). As involution proceeds, bottle cells subse-
quently form in more lateral and ventral marginal regions, drawing the entire circum-
ference of the involuting marginal zone inside the embryo. Once internalized, the
bottle cells change shape once again and spread to form the roof of the primitive gut,
the archenteron (Hardin and Keller 1988 ). The extent of mesoderm internalized by
involution varies greatly in different amphibian species. In the salamander, newt as
well as in some frog species, the presumptive axial and paraxial mesoderm internal-
izes during neurulation by “subduction,” a coordinated delamination from the epithe-
lia (Smith and Malacinski 1983 ; Purcell and Keller 1993 ; Delarue et al. 1994 ; Shook
et al. 2002 ). During gastrulation, convergence movements bring cells from the lateral
regions to the dorsal midline. At the midline, they change their shape and align them-
selves with respect to the anterior–posterior axis (Solnica- Krezel 2005 ). These
movements and cell shape changes transform the embryo from a sphere to a rod.
7.3.3 Teleosts
Teleost fish represent about half of all known vertebrate species, and they follow a
common plan of embryonic development (Postlethwait et al. 2000 ). The eggs have
a large amount of yolk, like avian/reptilian embryos, and the cells sit on top this
yolk and undergo meroblastic cleavage. At the blastoderm stage, cells can be divided
into the deep cells, which will produce the embryo, and an outer layer of extraem-
bryonic cells called the enveloping layer (EVL), which produces a transient outer
layer called the periderm (Kimmel and Law 1985b; Iwamatsu 1994 ). At the begin-
ning of the blastoderm stage, the cells adjacent to the yolk fuse with each other and
W. Tseng et al.