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are scarce (Wise et al. 2009 ; Matsubara et al. 2014 ). Reasons for this can include
difficulty of establishing breeding colonies in the lab, small clutch sizes, fertiliza-
tion time uncertainty for wild-caught pregnant females, difficulty in culturing
embryos due to extreme temperature and humidity sensitivity, and difficulty isolat-
ing embryos because of embryo adhesion to the egg’s inner surface. However,
recent work has begun to overcome these obstacles (reviewed in Wise et al. 2009 ;
Matsubara et al. 2014 ). A method for culturing embryos of the Japanese striped
snake (Elaphe quadrivirgata) (Matsubara et al. 2014 ) is a major step forward. While
the earliest stage depicted in the study was a gastrula-stage embryo, it appeared
quite similar to a chick embryo at the same stage. The data focused on somitogen-
esis, but this method has promise for examining early cleavage stages in snakes.
Other studies have suggested various lizards, such as the leopard gecko
(Eublepharis macularius), as a model for development in that group (reviewed in
Wise et al. 2009 ) and presented staging series. So far though, the majority of these
focus on embryos in eggs that have already been laid, which is too late for charac-
terization of cleavage stages. However, easy husbandry of these animals, combined
perhaps with methods similar to those mentioned above for snakes and chicks, has
potential to further the study of cleavage patterning in this phylogenetically impor-
tant group of reptiles.
4.3.4.2 Early Cleavage Divisions in Mammals
Analogous to fish, amphibians, birds, and reptiles, mammalian zygotes initially
undergo a series of cleavage divisions following fertilization to produce an increas-
ing number of progressively smaller cells without changing the overall size of the
embryo. However, the introduction and optimization of in vitro fertilization (IVF)
and embryo culture techniques have revealed several notable differences in how
these early divisions occur between mammals and other vertebrate animals. First,
mammalian species exhibit rotational cleavage, whereby meridional division is
observed along the animal–vegetal axis in the first cleavage, but during the second
cleavage, the daughter cells can divide either meridionally or equatorially by divid-
ing perpendicular to the animal–vegetal axis (Gulyas 1975 ; Fig. 4.8). As a conse-
quence, each blastomere inherits equivalent cytoplasmic material from both the
animal and vegetal region at the two-cell stage and potentially differentially allo-
cated animal and vegetal portions when the embryo divides from two cells to four
cells (Gardner 2002 ). The type of second division each daughter cell undergoes
determines which of the four distinct classes (meridional–meridional, meridional–
equatorial, equatorial–meridional, or equatorial–equatorial) a four-cell embryo will
become, and this may impact both cell fate and developmental potential as previ-
ously suggested (Piotrowska-Nitsche and Zernicka-Goetz 2005 ). More specifically,
it has been shown that four-cell mouse embryos containing at least two blastomeres
with both animal and vegetal material are much more likely to develop to term than
embryos where all blastomeres have either only animal or only vegetal cytoplasmic
inheritance (i.e., the equatorial–equatorial class; Piotrowska-Nitsche et al. 2005 ).
A. Hasley et al.