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specific genes in the YSL, however, indicates that it expresses an essential ventral-
izing signal in addition to its ability to induce dorsal mesoderm (Fan et al. 2007 ; Sun
et al. 2014 ). Thus in teleosts, the extraembryonic YSL performs the same functions
during development that are fulfilled by the vegetal blastomeres in amphibians (Fig.
7.7b). The YSL fits all the criteria for Nieuwkoop Center. Signals from the YSL are
both necessary and sufficient to induce the shield, and the YSL does not contribute
to the embryonic axis, since it is an extraembryonic tissue.
7.5.3 Amniotes
7.5.3.1 The Node
The functional equivalent of Spemann's Organizer in amniotes is called the node. In
1875, Viktor Hensen (1835–1924) was studying the embryonic development of the
rabbit when he noticed an epithelial thickening at the anterior end of the primitive
streak in gastrulating embryo (Hensen 1876 ). Because of its appearance, he called
this structure the node, although in some texts it is referred to as the primitive knot.
No-tail/BrachyuryRNasea b
Fig. 7.7 Experiments to find the mesoderm inducing signals in zebrafish. (a) Zebrafish animal cap
explants differentiate into epidermis when cultured alone, but express mesodermal markers when
cultured on a free yolk cell (Ober and Schulte-Merker 1999 ). In untreated embryos, cells near the
margin express the pan-mesodermal marker, No-tail/Xbra. No-tail/Xbra expression is greatly
reduced when RNase is injected into the YSL, and an ectopic no-tail/Xbra appears when an ectopic
yolk cell is grafted on the animal pole of a host embryo. (b) Signals from the YSL induce meso-
derm and endoderm (small black arrows), and signals from the dorsal YSL induce dorsal meso-
derm and endoderm (large black arrow)
W. Tseng et al.