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Chapter 24
Reproduction and
Development
Fertilization
• Union of a haploid
sperm nucleus with a
haploid egg nucleus
• This results in a
diploid zygote with the
full number of
chromosomes
Two types of Fertilization
Internal Fertilization
External Fertilization
Internal Fertilization
• Takes place inside the body of
the female
• Less eggs are required
Ex: mammals and birds
Internal Fertilization
External Fertilization
• Eggs are fertilized outside the body of
the female
• Large number of eggs are required
• This type of fertilization is found in
an aquatic environment
Ex: fish, frogs, & salamanders.
(I) Embryonic Development
• In the early stages of development, the
organism is called an embryo
• The process of embryonic development
includes:
1. Cleavage
2. Gastrulation
3. Growth and Differentiation
(A)
Cleavage
• Cleavage is a series of mitotic
divisions where the cell increases in
cell number but NOT in cell size
• Cleavage converts a single fertilized
egg (zygote) into many cells
1) Formation of a zygote (fertilization)
2) Cleavage of the zygote (mitotic cell
division) 1 cell into 4 cells.
Stages of Cleavage
Stages of Cleavage
Stages of Cleavage
1st division (2 cells)
Stages of Cleavage
2nd division (4 cells)
Stages of Cleavage
3rd division (8 cells)
Stages of Cleavage
4th division (16 cells)
Stages of Cleavage
5th division (32 cells)
As cleavage continues, the cells form a
hollow ball-type structure filled with fluid.
At this point, the embryo is called a blastula
Stages of Cleavage
The Blastula (hollow)
Stages of Cleavage
Stages of Development
Gastrulation
Pinching
Stages of Cleavage
Ectoderm
Endoderm
(B)
Gastrulation
• When the blastula
reaches several
hundred cells,
gastrulation occurs
• In this stage, the cells
on one side of the
blastula push in and
form a two-layered
embryo called the
gastrula
Gastrula
• The gastrula consists
of 3 layers:
a) ectoderm - outer
layer of cells
b) mesoderm –middle
layer of cells
c) endoderm – inner
layer of cells
It is the three germ layers that give rise
to all the tissues and organs of an animal
Ectoderm
Mesoderm Endoderm
nervous system
bones and
lining of
muscles
digestive tract
lining of mouth,
nostrils, and
blood and blood lining of trachea,
anus
vessels
bronchi, and
lungs
epidermis of
reproductive and
skin, sweat
excretory system liver, pancreas
glands, hair, and dermis layer of
thyroid,
nails
skin
parathyroid
urinary bladder
(C) Growth and Differentiation
• Differentiation- series of changes that
transform the unspecialized embryonic
cells into specialized cells, tissues and
organs
• Growth- mot only are the number of
cells increasing, but the size of the cells
are increasing as well, and the embryo
as a whole starts to develop
(II)
Development
Two types:
External Development
Internal Development
External Development
• Development occurs outside the female body
a) in water:
1. Nourishment for the developing embryo is supplied
by the yolk stored in the egg
2. No parental care
3. Examples are fish and frogs
b) on land:
1. Production of egg and shell (protection)
2. SOME parental care
3. Developing embryo’s source of food is the yolk
4. Examples are birds and reptiles
Sunday April, 24th,
2001
Wednesday, May, 2nd,
2001
Early development
(cleavage)
Late development (differentiation)
External Aquatic Development
(B) Internal Development
• Development occurs inside the
female
• Two types:
Placental mammals
Marsupials
(II)
Development
External Development
External Terrestrial Development
Emperor Penguin Hatching
Structure of Egg
•
a)
b)
c)
d)
Consists of 4 membranes
outside of the embryo
chorion- outer-most
membrane, aids in gas
exchange
allantois- exchange of O2
and CO2. Also a storage site
for wastes
amnion- sac containing
amniotic fluid which
surround the embryo,
provides cushion and acts as
a shock absorber
yolk sac- source of food for
the embryo. Blood vessels in
the yolk sac transport food
to the developing embryo
Photo not available
(B) Internal Development
• Development occurs inside the
female
• Two types:
Placental mammals
Marsupials
Placental Mammals
• These are animals, like humans, that contain a
structure called the uterus where the embryo
develops
• Within the uterus, a specialized structure called
the placenta forms
• It is through the placenta that the exchange of
nutrients, wastes, and respiratory gases
between the embryo and the mother take place
• The umbilical cord, which contains blood
vessels, attaches the embryo to the placenta
Marsupials
• Are mammals that do not have a placenta
• The embryo is born at a relatively
premature stage, and completes its
development externally in a pouch that
contain mammary glands
• Examples are kangaroos and opossum