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Development
Axolotl
Chicken
Gametogenesis
The formation of gametes
occurs within the gonads
(ovaries and testes.)
Spermatogonia and
oogonia undergo mitosis
to produce primary
spermatocytes and
oocytes.
Primary gametocytes
undergo meiosis twice to
produce four spermatozoa
or a single ovum.
Fertilization
The haploid egg and haploid sperm
unite resulting in a diploid zygote.
Cell Cleavage
 Rapid divisions increase
cell numbers with no
increase in size
 1st Cleavage = 2 cells
 2nd Cleavage = 4 cells
 3rd Cleavage = 8 cells
 4th Cleavage = 16 cells
 5th Cleavage = 32 cells
(morula)
 Hollow ball of cells
(blastula) forms with a
fluid filled cavity in the
center (blastocoel)
Gastrulation
 Invagination
(denting inward) and
involution (rolling
inward) of cells
producing
blastopore.
 The 3 primary germ
layers are
established:
 Ectoderm
 Mesoderm
 Endoderm
Gastrulation in Aquatic Vertebrates
Ectoderm formed as cells from the animal
pole surround larger yolk cells which form
the endoderm.
Mesoderm formed at an involuting dorsal
lip at the blastopore (see models)
Gastrulation in reptiles, birds, and
mammals
The cells of the blastodisc which are adjacent to the yolk become
endoderm.
Mesoderm is formed at the involuting primitive streak (an elongated
blastopore) on the dorsal surface of the blastodisc.
Germ Layer Development
 Ectoderm
 Nervous system, cornea and lens of eye, epidermis of
skin, epithelial lining of mouth and rectum
 Mesoderm
 Circulatory system, segmented muscles, skeleton,
kidney, gonads, outer covering of internal organs,
lining of thoracic and abdominal cavities
 Endoderm
 Pharynx, liver, pancreas, lining of digestive tract,
lining of respiratory tract
Axolotl Development
Cleavage (2 cells)
Cleavage (4 cells)
Blastula
Early Gastrula
Late Gastrula
Early Neurula
Early Tail Bud
Late Tail Bud
Chick Embryo Development
4 day embryo removed from
the egg:
http://www.youtube.com/watch?v=Bzvpa1SaG
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9 day embryo inside egg:
http://www.youtube.com/watch?v=Tjeq58oLd
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Fertilized egg to a baby chick
http://www.youtube.com/watch?v=kwGkkOeo
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Candling: Interpreting what you see…
Freshly laid egg (air pocket and
yolk are visible): if it is over a week
old it is not fertile
Fertile egg at approximately 5 days
old. Notice the faint blood vessels.
Opaque egg: the chick has
developed to the point that you
can’t see through the egg at all.
This egg is opaque, but it has a
large, off-center air pocket. It could
contain a dead-in-shell chick, or it
could be an old rotten egg.
Chicken Egg Candling
• Unfertilized Chicken Egg
• Fertilized Chicken Egg
Today’s Tasks
• Station 1: Model amphibian development
– Sketch 1 stage & label layers & other identifiable structures
• Station 2: Preserved and live frog embryos
– 1 sketch any stage
• Label developmental stage and identifiable structures
– Discuss questions in your discussion
• Station 3-4: Live axolotl, frog & killifish embryos
– 2 sketches of axolotls at different developmental stages
– 2 sketches of killifish at different developmental stages
• Label developmental stages, and any of the following you can find: yolk,
head, tail, dorsal/ ventral sides, eyes, blood vessels, heart
– Discuss questions in your discussion
• Station 5: Chick Embryos
– 2 sketches at different developmental stages
• Refer to description card to label all identifiable parts
– Discuss questions in your discussion