Animal Development
Chapter 47
Development
 Preformation – Idea that egg contains
a miniature adult that grows only in size
during devel.
 Epigenesis – Animal emerges from a
formless egg (grows and differentiates).
 Microscopy has led to the overwhelming
acceptance of epigenesis as the pattern
of development in animals.
Fertilization
 Combination of egg nucleus and sperm
nucleus.
 Activation of egg to begin development
of zygote.
Internal  Fert. takes place inside
body of female (ie: humans)
 External  Fert. takes place in
external environment (ie: fish)

The Acrosomal Reaction
 When it contacts the jelly coat of the
egg, the sperm releases digestive
(hydrolytic) enzymes from acrosome.
 Sperm digest jelly coat and membranes
fuse.
 Sperm nucleus is released into the egg.
 Fast block to polyspermy  egg
depolarizes (Na+ in) to prevent double
fertilization.
Cortical Rxn (Slow block to polyspermy)
 Progressive
release of Ca+2 by the
egg’s ER causes cortical granules to
release enzymes
 The enzymes in effect harden the
egg’s membrane (fertilization
membrane)
 This hard membrane is not penetrable
by any more sperm, so only 1 sperm
fertilizes the egg
Fertilization in Sea Urchin (external)
Egg Activation
 High levels of Ca2+ also trigger the egg
to increase cellular respiration and
protein synthesis
 As this takes place, the nuclei of the
egg and the sperm fuse with one
another
 Diploid zygote nucleus is formed
 Cell division and DNA replication begin
as the zygote develops
Mammal Fertilization (internal)
 Capacitation  Secretions by female
enhance the function of sperm in
internally fertilizing mammals
 Zona pellucida recognizes the binding
sperm
 Acrosomal and cortical reactions both
occur in mammals
 Fast and slow polyspermy blocks similar
to sea urchins
Mammalian Fertilization
 Entire sperm enters egg and the flagella
divide to produce spindle of dividing egg
 Egg and sperm nuclei do not fuse
immediately
 Share spindle in zygotes initial division.
 Appear first as diploid zygote as
daughter of 1st mitotic division
Sea urchins (external) fused DNA
immediately
Mammalian Fertilization
Cleavage
 Rapid cell division that changes the
large, single-cell zygote into a large ball
of much smaller cells (blastomeres)


Cells are smaller because the G1 and G2
phases are generally skipped
Blastomeres each contain different
cytoplasmic molecules (determine fate
of cells?)
Cleavage in Sea Urchins
Most animals (not
mammals) exhibit a
patterned distribution
of materials
 Vegetal Pole  High
yolk
 Animal Pole  Low
yolk, often anterior of
animal
 Cleavage is more
rapid in animal pole

Polarity
Cleavage
In sea urchins and
frogs, first two
divisions vertical,
third horizontal
 Eight-celled
embryo with two
tiers of four cells is
what continues
with cleavage

Cleavage  Morula Stage


Cleavage continues
and the morula is
formed
The morula is a
solid ball of cells
Cleavage  Blastula Stage


A fluid filled cavity
(blastocoel)
forms within the
morula
Creates a hollow
ball of cells
(blastula)
Meroblastic/Holoblastic Cleavage
Meroblastic Cleavage
 Birds (plentiful yolk) restrict cleavage to
the animal pole of the zygote
Holoblastic Cleavage
 Sea urchins, frogs (less yolk) show
complete division of the egg
Gastrulation
 Development of a three-layered embryo
(gastrula) with a primitive gut from the
blastula
 Three Embryonic Germ Layers
 Ectoderm
 Endoderm
 Mesoderm
Sea Urchin Gastrulation
 Begins at the vegetal pole where
individual cells enter the blastocoel as
mesenchyme cells
 Rest of cells buckle in to form the
archenteron (invagination)
 Open end of archenteron will form the
anus and the other end the mouth
 Thus, the archenteron becomes the
digestive tube (mouth to anus)
Sea Urchin Gastrulation
Organogenesis (differentiation)
 Ectoderm germ layer gives rise to:
 Epidermis of skin, and its derivatives
 Epithelial lining of the mouth and
rectum
 Cornea and lens of the eyes
 The nervous system; adrenal medulla;
tooth enamel; epithelium of the pineal
and pituitary glands
Organogenesis
 Endoderm germ layer becomes:
 The epithelial lining of the digestive
tract (except the mouth and rectum).
 The epithelial lining of the respiratory
system.
 The pancreas; thyroid; parathyroids;
thymus; the lining of the urethra,
urinary bladder, and reproductive
systems
Organogenesis

Mesoderm germ layer becomes:
 The notochord
 The skeletal and muscular systems
 The circulatory and lymphatic systems.
 The excretory system
 The reproductive system (except germ
cells)
 And the dermis of skin; lining of the
body cavity; and adrenal cortex
Organogenesis
Amniotic Eggs
 Allows terrestrial organisms to
reproduce in a dry environment
 Organisms
is kept in a fluid filled
environment
 Mammals
and birds both have
amniotic eggs
Avian Development
Avian Development
Avian Development
Extraembryonic Membranes
• yolk sac providing nutrients to the
embryo
• Amnion encloses the embryo in a
fluid-filled amniotic sac (protects the
embryo from dessication)
• Chorion cushions the embryo
• allantois disposal sac for uric acid
(urine)
Amniotic Egg (Bird/Reptile)
Human Amniotic Development
Human embryos develop in amniotic eggs
the way birds and reptiles do, except they
are implanted in the uterus, not externally
developing
 There is more exchange of materials b/t
mother and offspring (blood cells for
immunity, nutrients, waste)
 ie: placental mammals (humans)
 ie: marsupial mammals (kangaroos)
