213the use of specialized molecular and genetic approaches has largely limited the study
of axis formation to a few tractable vertebrates, notably the mouse, chicken, Xenopus,
and zebrafish. However, these species are all fairly evolutionarily derived representa-
tives of their respective groups, making inference of primitive vertebrate characters
problematic. With the growing ease of high-throughput genome analyses, stem cell
technology and programmable genome editing, the barriers to performing compara-
tive molecular and genetic studies are becoming increasingly reduced, potentially
heralding a return to a broad comparative approach to vertebrate development.
In the context of the egg-to-embryo transition, the formation of the body axis is
perhaps a defining process, since early developmental processes become organized
into a unit comprising an individual. Indeed the idea of individuality in twinned
embryos was an inspiration for Spemann to begin studying the embryological mech-
anisms of axis formation (Hamburger 1988 ), and remains relevant in current bioeth-
ics arguments regarding human embryos. This chapter shall review the core concepts
relating to the origins and patterning of the axis, focusing on recent advances in
understanding intracellular reorganizations, intercellular signaling events and cellu-
lar migrations. Emphasis has been given to recent molecular advances in the context
of first discoveries and initial functional studies. Many of the ideas in this chapter
have been extensively reviewed separately in the context of certain organisms, mol-
ecules or individual processes, but this chapter will attempt to tie these together to
generate a more unified picture of axial development throughout the vertebrates.
6.2 Origins of Axial Polarity in the Egg and Early Embryo
The process of determining the initial plane of bilaterality and axis formation was
first examined closely in the amphibians, where the gray crescent served as marker
of the future axis (Fig. 6.2). Like most vertebrate eggs, amphibian eggs are initially
symmetrical about the animal–vegetal axis (axisymmetrical), with the animal pole
being the site of polar body extrusion, by definition, and more darkly pigmented.
The vegetal pole is less pigmented and more concentrated with yolk. Using local-
ized fertilization of frog eggs (Rana (Lithobates) spp.), Newport and Roux showed
that the meridian of sperm entry coincided with the embryonic midline (and often
the first cleavage plane), with the dorsoanterior axis forming opposite the sperm
entry side (Newport 1851 , 1854 ; Roux 1885 , 1887 ). Importantly, Roux noted an
apparent shifting of the animal–vegetal axis toward the sperm entry point, along
with the appearance of a lightened area in the pigment on the opposing side. This
feature formed before first cleavage and indicated the axial plane of the embryo,
irrespective of the cleavage plane, which could be highly variable relative to sperm
entry, depending on species (Roux 1887 , 1888 ). Later studies confirmed these
observations, further showing that the position of this “gray crescent”^1 strongly
(^1) The nomenclature of the gray crescent has been quite variable. Roux ( 1888 ) referred to this fea-
ture as a “crescent-shaped gray seam” (halbmondförmigen grauen Saumes). Morgan and Tsuda
6 Vertebrate Axial Patterning: From Egg to Asymmetry