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Development
Types of eggs
Cell cleavage
radial vs. spiral
Determinate vs. indeterminate development
Blastulae
Gastrulation
Mesoderm formation
Coelom formation
Direct vs. indirect development
Types of Eggs
Animal pole - tends to become
other regions of the embryo
Vegetal pole - tends to become
digestive system
Types of Eggs
Isolecithal - scanty
yolk distributed
uniformly through
egg
Centrolecithal - yolk
distributed in center
around nucleus
Telolecithal - yolk
concentrated at
vegetal pole
Cell cleavage
A fertilized egg is a zygote. The zygote splits and the resulting
cells are blastomeres.
Holoblastic cleavage cleavage planes pass all the
way through the cell.
Occurs in isolecithal and
weakly telolecithal zygotes.
Meroblastic cleavage cleavage planes do not
pass through dense yolk
so blastomeres are not
separated from each other.
Cell cleavage
First cell division is parallel
to the animal-vegetal pole
(longitudinal division)
Second cell division is
also parallel to the
animal-vegetal pole
Third cell division
is perpendicular to
the animal-vegetal
pole (transverse
division),
sometimes
resulting in
micromeres and
macromeres
Radial vs. spiral cleavage
Radial cleavage
Spiral cleavage
First quartet of cells
Second quartet lies
directly on top of
first quartet
Second quartet lies in
depressions between
first quartet. Top cells
are micromeres and
bottom cells are
macromeres.
Spiral cleavage
• E.B. Wilson developed a numbering
scheme to follow individual cells
through spiral cleavage.
• Macromeres and micromeres are
grouped into quartets and numbered
according to first quartet ancestor
and number of cell divisions.
• Allows embryologists to follow cell
fates and compare development in
various taxa.
Determinate vs. indeterminate development
Determinate development - fates of early blastomeres (2-4
cell stage) fixed. Embryo will not develop properly if cells
are removed.
General uniformity in fate of cells for spiral, determinate
development:
ectoderm from first quartet of cells
entoderm from 4a, 4b, 4c, and 4Q cells
mesoderm from 4d cell
Indeterminate development - cell fate is determined later in
development. Blastomeres can be separated early and each
separate cell develops normally.
Blastula types
Coeloblastula formed
from isolecithal eggs
that undergo radial
cleavage
Stereoblastula formed
from spiral cleavage
Discoblastula formed
from meroblastic
cleavage
Periblastula formed
from centrolecithal
eggs
Gastrulation
Development of a blastula into a multilayered embryo
Forms embryonic germ layers which will
become different tissues:
entoderm becomes gut
ectoderm becomes skin and nervous system
mesoderm becomes muscles, circulatory
system, and internal support system
When an embryo has all three of these layers it is triploblastic.
If it has only two, it is diploblastic.
Gastrulation
Coeloblastulae often
gastrulate by invagination.
Cells near the vegetal pole
grow inward leading to an
archenteron and a
blastopore. The inner
cells are entoderm and the
outer cells are ectoderm.
Gastrulation
Some coeloblastulae
undergo ingression in
which cells near the
vegetal pole grow to fill
the blastocoel. This leads
to a solid gastrula called a
stereogastrula.
Gastrulation
Rarely, cells of the
blastula divide to form
cells just below them in
delamination.
Gastrulation
Stereoblastulae that result
from holoblastic cleavage
generally undergo
epiboly. Cells from the
animal pole grow over the
rest of the blastula. The
archenteron occurs
secondarily.
Gastrulation
Discoblastulae often
gastrulate by involution.
Cells around the disc
proliferate and grow
under the disc. The yolk
is still present.
Mesoderm formation
True mesoderm is derived from entoderm.
If a middle layer is derived from ectoderm, it is
ectomesoderm and the animal is considered diploblastic.
Schizocoely - In organisms with
spiral cleavage, the 4d micromere
proliferates between the
archenteron and the ectoderm to
form a solid mass of mesoderm.
Enterocoely - In other organisms
entoderm pouches off the archenteron
and becomes mesoderm.
A true coelom is completely surrounded by mesoderm. Some
organisms have coeloms that are not surrounded by mesoderm.
These are called pseudocoelomates or blastocoelomates.
Indirect development
Gamete ---> free living larvae ---> adult
Metamorphosis brings huge
changes in structure and function.
Much mortality at this time.
Larvae are induced to
metamorphose by cues such as
light, temperature, substratum, and
conspecifics.
Some larvae can delay
metamorphosis for days to months.
Indirect development
Gamete ---> free living larvae ---> adult
Advantages 1) young do not compete with
adults for food or habitat.
2) not much energy put into eggs
or care of young.
Disadvantages 1) high levels of mortality (less
than 1% typically survive)
Direct development
No free-living larvae. Larvae are brooded or
encapuslated and hatch as juveniles.
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Advantages 1) less mortality.
Disadvantages 1) more energy put into eggs and
care of young.
2) young compete with adults but
can find different
microhabitats.
Common in freshwater and terrestrial organisms. Habitats stressful for
eggs and larvae
Mixed development
Larvae are brooded or encapuslated for some part of their
development. Hatch as late-stage free-living larvae.