DEVELOPMENT
* occurs after both the male and female gametes (sperm and eggs) fuse-FERTILIZATION
* the process by which the fertilized egg, the ZYGOTE develops from a single cell
into an adult multicellular organism
Development
I. ANIMAL DEVELOPMENT
A) Embryonic Development
* the changes a zygote undergoes as it changes into an embryo
1) cleavage-* a series of mitotic cell divisions that increase the number of cells but not the size
of the cells
* actually, the cell size becomes smaller
* leads to the formation of a hollow ball called the BLASTULA
2) gastrulation-* in certain animals, the blastula becomes indented, forming a GASTRULA
* the gastrula has 3 layers, sometimes called germ layers:
* ENDODERM-the inner layer
* ECTODERM-the outer layer
* MESODERM-the layer in between the endoderm and the ectoderm
3) differentiation-* the different germ layers in the gastrula develop into special cells, tissues, and
organs of multicellular animals
ECTODERM: nervous system and the outer skin
MESODERM: muscles, skeleton, circulatory and excretory systems, gonads
ENDODERM: linings of the digestive and respiratory systems, as well as parts of
the liver and pancreas
a great look at the
early stages of
embryonic
development
awesome site from
Univeristy of Utah's
Genetic Science
Center on stem cells;
look at their main
page too!
Tim and Moby discuss
stem cells.
* these cells are called embryonic stem cells and can become all types
of cells
* during differentiation, the stem cells become more specialized for their function
* scientists are trying to revert mature, specialized cells back to the stem cell phase
(dedifferentiation--see pic below) so they can help out individuals by turning them
into a needed type of tissue! examples:
1. healing spinal cord injuries
2. growing new heart tissues
3. forming new muscles for trauma patients
4) growth-* an increase in size or cell number as differentiation continues
* early development consists chiefly of the differentiation and growth of cells and
tissues
The above embryonic development can occur either outside (externally) or inside
(internally) the mother.
B) External Development
* the development of the embryo outside the mother
1) in water
Development
* in fish and amphibians, the eggs are fertilized externally and then develop
externally as well
* the embryo's food source is the yolk in the egg
* gas exchange for the embryo occurs by diffusion through the thin egg with the
watery environment
* water also acts as a shock absorber for protection
2) on land
* after being fertilized internally, the eggs of birds, reptiles, and a few mammals (like
the duck billed platypus) lay their eggs to develop on land
ADAPTATIONS OF EGGS
a) shell-outermost layer which provides protection and a favorable environment
b) yolk-food source of egg; produced by uneven cytoplasmic division during
oogenesis (meiosis)
c) different membranesi) chorion- the outermost membrane which is under the shell; contains the other
membranes
ii) amnion- contains AMNIOTIC FLUID which cushions the embryo and prevents
cells from sticking to the shell
iii) yolk sac- surrounds the yolk; blood vessels penetrate the yolk sac to carry food to
the developing embryo
iv) allantois- functions as a respiratory surface (exchange of gases); also acts as a
storage site for the nitrogenous waste produced, uric acid
C) Internal Development
* the development of an embryo inside the mother
1) placental mammals
* mammal embryos develop within the UTERUS after internal fertilization
* the PLACENTA is a special organ made of both embryonic and maternal (mother)
tissues
* the placenta is the place where the exchange of nutrients, wastes, and respiratory
gases occurs by diffusion
* an UMBILICAL CORD with blood vessels attaches the developing embryo to the
placenta
* the bloodstreams of the embryo and the mother never mix!
2) marsupial mammals
* these mammals have eggs that are fertilized internally
* they develop internally for a short period of time without a placenta
* the embryo is then born VERY EARLY IN ITS DEVELOPMENT!
* then, the born embryo continues its development externally in a pouch which
contains the mammary glandssupplies the nutrients to the young " kangaroos,
opossum, and koala bears (not shown) are examples of marsupials
II. PLANT DEVELOPMENT
* both meiosis (the production of sex cellsgametes) and fertilization (the fusion of gametes)
occur in the flower, a plant structure specialized for
reproduction
Development
A) The Flower
* may contain both male (stamen) and female (pistil)
reproductive organs...
* some species might have only one or the other
* flowers might have other accessory structures
such as:
petals- used to attract insects
sepals- the leaf of the flower
1) the stamen-* composed of the anther and filament (which looks
like a wire)
* the diploid (2n) cells of the anther produce the
monoploid (n) pollen grains, which is the male
gamete
* MEIOSIS OCCURS IN THE STAMEN
2) the pistil-* composed of the stigma, style, and the ovary
a) stigma- catches pollen (sticky-stigma) is the
upper part of the pistil
pollination--so that is why they talk about the 'birds
and the bees'...they mean plant sex!
b) style- connects the stigma to the...
c) ovary- monoploid (n) eggs are produced here in
ovules; MEIOSIS OCCURS IN THIS PART OF THE
plant sperm--I mean
pollen!
PISTIL
B) Pollination
* the transfer of pollen grains from the anther (part of
the stamen) to the stigma (part of the pistil)
* pollination may be accomplished by wind, insects,
or birds that carry the pollen
* colored petals and nectar can attract different
organisms to help pollination occur
1) Self-Pollination
* the transfer of pollen on the same plant
* can occur in the same flower or from flower to
flower of the same plant
* uses the genetic information from that same plant
but is still sexual reproduction! (two cells are
involved)
2) Cross-Pollination
* the transfer of pollen of one plant to another plant
entirely
* allows for the combination of characteristics from
both "parent plants"
* only one problem...the environment is usually very
dry
* pollen grains have thick walls to help prevent water
loss (dehydration) of the nucleus (found inside the
pollen grain)
C) Fertilization and Development
* after pollination, the pollen grain germinates (grows) on the stigma
* as it grows, it forms a pollen tube which extends into the ovary of the
pistil
Development
* the sperm now forms from the nucleus of the pollen and travels down the
pollen tube
1) Fertilization in the Flower
* when the sperm fertilizes the egg in the ovule (part of the ovary) a zygote
is formed
* the zygote develops by mitosis into an embryo
* the embryo and now matured ovule is called a SEED
a) seed coat- a tough protective outer layer that develops from the walls of all about seeds
the ovule
b) fruit- the entire ripened ovary
2) Development of Embryos
* the seed has 3 different parts:
a) hypocotyl- becomes roots and lower stem of adult plant
b) epicotyl- becomes upper stem and leaves of adult plant
c) cotyledon- stored food for germinating plant
3) Germination and Growth
*seeds grow under suitable environmental conditions such as:
* sufficient moisture
* proper temperature
* sufficient oxygen for aerobic respiration--NRG!
* seeds will grow into mature plants
* growth occurs at regions called MERISTEMS