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The evolutionary origin of the YSL is unclear. Reptile and avian embryos have a
YSL, but it is not likely that these are homologous to the teleost YSL, as has been
suggested (Nagai et al. 2015 ). In reptile and bird embryos, the YSL forms during the
incomplete meroblastic divisions of the cleavage stage embryo, whereas the teleost
YSL forms at a later stage by the fusion of blastoderm cells. Although lamprey and
bicher embryos have large extraembryonic yolk cells, the garpike is the most basal
vertebrate with a true YSL (Long and Ballard 2001 ). The garpike (Lepisosteus)
YSL may be homologous to the teleost YSL in structure and function, but it has also
been proposed that the teleost YSL is evolved from the primary hypoblast in bowfin
(Amia) embryos, an intermediate species between Lepisosteus and teleosts (Long
and Ballard 2001 ; Cooper and Virta 2007 ; Comabella et al. 2014 ). The presence of
extraembryonic yolk cells in basal vertebrates, teleosts, gar, reptiles, and birds, in
addition to the formation of the trophoblast in mammals indicates that a common
strategy in vertebrate development is to segregate extraembryonic and embryonic
lineages before gastrulation. A major, and potentially interesting exception is the
amphibians, in which the yolk is internal to the cells.
7.4 The Fate Map of the Vertebrate Blastoderm
7.4.1 The Amphibian Fate Map
Pander and von Baer understood that the three germ layers generate different organs
in embryo, but it was not until the early twentieth century that the first fate maps of
vertebrate embryos were made. In 1925, the German embryologist Walther Vogt
(1888–1941) developed the first technique to heritably label embryonic cells and
their descendants (Vogt 1925 ). He used dried chips of agar stained with Neutral Red
or Nile Blue to physically mark the surface of a particular region of the blastula or
gastrula stage embryo. The residue of the agar chip would stick to the surface of the
cells, and be inherited by its daughters at each subsequent cell division. He could
then determine which regions of a blastula stage embryo produced a specific tissue
or organ. Amphibians were ideally suited for this analysis, because there is very little
cell mixing at the early stages and the future dorsal axis is apparent during the first
cell cycle with the formation of the grey crescent. Vogt constructed detailed fate
maps of a wide variety of urodele and anuran embryos, including axolotls, newts,
and toads (Vogt 1929 ). Osamu Nakamura (1911–) revised this fate map in 1938 and
again in 1942 with improved vital dye staining techniques (Nakamura 1938 , 1942 ).
Other scientists used similar surface labeling techniques to fate map the South
African Clawed Frog, Xenopus laevis and other amphibian embryos (Pasteels 1942 ;
Keller 1975 ). There are, however, several limitations of the technique. The mark is
diluted over successive generations of cell divisions, and can be transferred to neigh-
boring cells by contact. Deep cells of the embryo cannot be lableled in this manner,
making it impossible to map the origin of internal organs and tissues with this tech-
nique. These weaknesses were addressed by the development of techniques to inject
embryos with Horse Radish Peroxidase (HRP), an enzyme that can be detected even
W. Tseng et al.