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predicts the area in which the future dorsal lip of the blastopore would form (Morgan
and Tsuda 1894 ; Schultze 1899 ; Roux 1903 ; Morgan and Boring 1903 ; Brachet
1904 ). These classical embryological observations established that the dorsal axis
and bilateral symmetry were determined upon fertilization of the egg and likely
occurred prior to the first division, thus disputing the long-held idea that the cleav-
age itself was determinative.
Despite the important nature of the connection between the gray crescent and the
dorsal axis, the mechanisms of gray crescent formation and function remained elu-
sive for many years. In the first comprehensive effort to understand the cellular
changes underlying gray crescent formation, Ancel and Vintemberger ( 1948 ;
reviewed in Clavert 1962 ) examined the movement of electrocautery wounds that
were made either in the deep yolk or on the surface of frog eggs. The motion of
these markers revealed that outer egg cortex moves dorsally relative to the station-
ary deeper yolky cytoplasm. Although other models were also considered, such as
asymmetric cortical contraction, a later series of marking and egg manipulation
studies in Xenopus largely substantiated and clearly documented the cortical rota-
tion model (Vincent et al. 1986 ; Vincent and Gerhart 1987 ).
( 1894 ) used the term “white crescent,” whereas Morgan and Boring ( 1903 ) used “grey crescent”
[sic], translated as “graue Feld” (gray field). Later this became universally referred to as the gray
crescent/grauer Halbmond/croissant gris.
g.c.s.e.p.anvegfuture
ventralfuture
dorsalFig. 6.2 Gray crescent formation in amphibians. Top panel, diagram of an amphibian egg (e.g.,
Rana) before (left) and after fertilization (right). The heavily pigmented animal pole (an) and the
paler vegetal pole (veg) are indicated. After fertilization, corticocytoplasmic movements opposite
to the sperm entry point (s.e.p.) result in the appearance of the gray crescent (g.c.) on the prospec-
tive dorsal side. Bottom panel, images of a Rana egg at fertilization (a), and at 20 min post-
fertilization, showing the gray crescent (b; dorsal view, arrow). Bottom panel reproduced from
Rugh ( 1951 )
D.W. Houston