EEB263 – Comparative Vertebrate Anatomy
Quiz 2 – Review
• The union of two mature sex cells, or gametes, constitutes fertilization. The
male gamete is the sperm and the female the ovum, or egg. Both of the
gametes carry genetic material from each parent, and both are haploid
(containing half the chromosomes of each parent). The sperm’s passage
through the outer layers of the ovum activates embryonic development.
• Although an egg can be very large, as is a chicken egg, it is but a single cell
with a nucleus, cytoplasm, and cell membrane, or plasma membrane.
• While still in the ovary, the ovum accumulates vitellogenin, a transport
form of yolk formed in the liver of the female and carried in her blood. Once
in the ovum, vitellogenin is transformed into yolk platelets consisting of
storage packets of nutrients that help support the growing needs of the
• The quantity of yolk that collects in the ovum is specific to each species, in
general there are 3 types:
1. Microlecithal: Slight amount
2. Mesolecithal: Moderate amount.
3. Macrolecithal: Enormous amount.
• The distribution of yolk in the ovum can also be categorized into 2 types:
1. Isolecithal: Even distribution.
2. Telolecithal: Concentrated at one pole
• When yolk and other constituents are unevenly arranged (telolecithal), the
ovum shows a polarity defined by a vegetal pole, where most yolk resides,
and an opposite animal pole, where the prominent haploid nucleus resides.
• Repeated mitotic division of the zygote occurs during cleavage. The embryo
experiences little or not growth in size, but the zygote is transformed from a
single cell into a solid mass of cells called the morula. Eventually the
multicelled and hollow blastula forms. The blastomeres are the cells
resulting from these early cleavage divisions of the ovum.
• The first cleavage furrows appear at the animal pole and progress towards
the vegetal pole.
• In microlecithal eggs of amiphioxus and placental mammals, cleavage is
holoblastic –mitotic furrows pass successfully through the entire zygote
from animal to vegetal pole. After the first few furrows pass through,
subsequence furrows perpendicular to these develop until a hollow ball of
cells form around an internal fluid-filled cavity. Structurally, the blastula is
the hollow ball around the internal blastocoel cavity.
• In mesolecithal or microlecithal egg, cell division is impeded, mitotic
furrowing is slowed, only a portion of the cytoplasm is cleaved, and cleavage
is said to be meroblastic.
• In extreme cases, such as in the eggs of many fishes, reptiles, birds and
monotremes, meroblastic cleavage becomes discodial because extensive yolk
material at the vegetal pole remains undivided by mitotic furrows and
cleavage is restricted to a cap of dividing cells at the animal pole.