159general rule of egg size and type of cleavage is unknown. However, blastula embryos
for E. coqui exhibit a distinct population of nutritional vegetal cells that are not
destined to become endodermal tissue (or any other embryonic cell type) (Buchholz
et al. 2007 ; Fig. 4.14b), and, as mentioned above, the presence of such nutritional
cells may represent an important step in the evolutionary transition from holoblastic
to meroblastic cleavage (Buchholz et al. 2007 ; Elinson 2009 ).
Thus, it appears that holoblastic cleavage is the ancestral pattern of cleavage in
craniates. Meroblastic cleavage, in which only a portion of the embryo is made up
of dividing blastomeres, has evolved independently at least five times within this
group. This evolution is sometimes, though not always, correlated with increased
egg size. Meroblastic cleavage evolution in teleosts is a significant exception and
has been shown to involve two evolutionary changes, loss of bottle cells and fusion
of yolk into a single mass. The presence of a population of nutritive cells, whether
ancestral or derived in a lineage, may be a key innovation on the way toward mero-
blastic cleavage. These innovations likely conferred a selective advantage, possibly
an increased ability to implement subcellular programs required for early
embryogenesis.
4.6 Conclusions
In this chapter, we have described mechanisms underlying patterns of cell cleavage
arrangement in early vertebrate embryos. Much of our knowledge on this topic stems
from studies in tractable developmental systems such as amphibians and teleosts.
Fig. 4.14 Intermediates between holoblastic and meroblastic cleavage patterns. (a) Partially
meroblastic cleavage in bowfin fish (A. calva). The yolk-rich vegetal region ceases to cleave after
the formation of 16 cleavage furrows. (b) Formation of nutritional endoderm in the Costa Rican
tree frog (E. coqui). Yolk-rich vegetal cells divide although do not eventually become part of the
embryo proper (Buchholz et al. 2007 ). Diagrams reproduced from Ballard (1986a, b), in panel (a)
and Buchholz et al. ( 2007 ) in panel (b), with permission
4 Vertebrate Embryonic Cleavage Pattern Determination