* 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

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

* 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.


stem cells--what are they and why are they so darn cool
a great look at the early stages of embryonic development has a nice walkthrough of development
awesome site from Univeristy of Utah's Genetic Science Center on stem cells; look at their main page too!
Cold Spring Harbor Laboratory has a nice overview of stem cells
Tim and Moby discuss stem cells.
see these insects in their different stages of life

B) External Development
* the development of the embryo outside the mother

1) in water

* 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


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 membranes-

i) 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 glands
supplies the nutrients to the young " kangaroos, opossum, and koala bears (not shown) are examples of marsupials


* both meiosis (the production of sex cells- gametes) and fertilization (the fusion of gametes) occur in the flower, a plant structure specialized for reproduction

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

2) the pistil--

* composed of the stigma, style, and the ovary
a) stigma- catches pollen (sticky-stigma) is the upper part of the pistil

b) style- connects the stigma to the...

c) ovary- monoploid (n) eggs are produced here in ovules; MEIOSIS OCCURS IN THIS PART OF THE 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)



plant sperm--I mean pollen!
pollination--so that is why they talk about the 'birds and the bees'...they mean plant sex!

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

* 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 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


all about seeds