139Study of chick cleavage stages reveals some striking similarities to groups out-
side amniotes. For example, evidence suggests that zygotic gene activation in the
chick occurs around the seventh or eighth cell division (precision is difficult due to
cell cycle asynchrony). This places the midblastula transition somewhere between
cycles 7 and 9, quite similar to the timing observed in zebrafish and Xenopus (Nagai
et al. 2015 ). These researchers also find evidence of a yolk syncytial layer in the
chick, though it is unclear if it plays a regulatory role similar to the YSL in zebrafish
(see Chap. 7 ). The division of a blastomere with the cleavage plane parallel to the
blastoderm, resulting in an inner cell and an outer cell that will go on to different
fates, is reminiscent of a similar process in Xenopus (Sheng 2014 ). This evidence,
combined with overall morphological similarity of cleavage-stage embryos in birds,
reptiles, and teleost fish, suggests deep ancestry or convergent evolution of many
characteristics of cleavage-stage embryonic development in vertebrates (Nagai
et al. 2015 ).
Reptiles warrant a mention here. They are an important group to study to under-
stand evolution of embryonic cleavage patterning in vertebrates and are known to
undergo meroblastic cleavage. Unfortunately, data on their earliest cleavage stages
Fig. 4.7 Cleavage pattern in the early chick embryo. Early cleavage in chick embryos during the
early to mid-cleavage stages (EGKI–EGKIV) tends to exhibit a perpendicularly alternating pat-
tern, diagrammed in (a) and visualized through scanning electron micrographs in (b). (c) Diagram
of a side view of the animal pole of the embryo, depicting meroblastic cleavage, with cellularized
cells in blue and the number of layers indicated by numbers. Reproduced from Lee et al. ( 2013 ) (a)
and Nagai et al. ( 2015 ) (b, c), with permission
4 Vertebrate Embryonic Cleavage Pattern Determination