The gametes, the sperm and the egg, are both highly specialized haploid cells that will combine chromosomes to produce the diploid zygote. The union of the sperm with the egg activates the egg, triggering the onset of embryonic development. The mammalian egg is cloaked by follicle cells that were released with the egg during ovulation. The sperm must migrate through this before it reaches the zona pellucida, the extracellular matrix of the egg. To get there the acrosome of the sperm (tip) releases hydrolytic enzymes to the zona pellucida enabling sperm to enter egg. The zona pellucida is made of 3 different glycoproteins. The protein ZP3 functions as a sperm receptor. In non mammals the acrosomal reaction releases hydrolytic enzymes that enable an elongating acrosomal process to penetrate the jelly coat of the egg and to bind to the vitellin

In many species only the zygote is totipotent, can be separated and have equal developmental potential. The first cleavage divides the cytoplasm so that only certain parts will give rise certain parts of the embryo. In contrast mammalian cells remain totipotent until they become arranged in a trophoblast. In the eight cell stage, each cell can be divided and bring about an individual.

The development of limbs is one type of pattern formation. A limb bud consists of a core of mesoderm tissue covered by a layer of ectoderm. There are organizer regions present in all vertebrate limbs. One type is the apical ectodermal ridge (AER), a thickened area of ectoderm at the tip of the bud. The AER is required for the outgrowth of the limb along the proximal-distal axis and for patterns along its axis. The cells of AER produce fibroblast growth factor, which appears to be the growth signal that promotes limb bud outgrowth. The second major limb-bud organizer is the zone of polarizing activity (ZPA). The ZPA is located where the posterior side of the bud is attached to the body. The ZPA is necessary for proper pattern formation along the anterior-posterior axis of the limb. Cells nearest to the ZPA give rise to the posterior structure and those farthest away form the anterior structure. ZPA secretes morphogens, the most important being a protein growth factor called Sonic Hedgehog. Production of this protein in the wrong part of the body sometimes results in an extra limb.

All vertebrates require an aqueous environment to develop and terrestrial animal evolved to suit reproduction in dry environments. Either within a shell or a uterus, birds, reptiles and mammals are surrounded by a fluid filled pouch called the amnion.

Various regions of the three germ layers develop into rudimentary organs during organgenesis. Folds, splits, and condensation are the first evidence of organ building. The first organs to be built in chordates are the neural tube and the notochord, the skeletal rod characteristic of all chordate embryos. The neural tube forms the central nervous system, the brain and spinal cord. The notochord is formed by condensation of the mesoderm and is later condensed even more into separate blocks called somites. Somite form the backbone vertebrates and muscles associated with axial skeleton. As organgenesis continues the organ become more and more refined. The neural crest, unique to vertebrate embryos, pinches off ectoderm and migrates to other parts of the cell; forming pigment cells of skin, teeth, and other peripheral components.

Cadherins are an important cell-to-cell adhesion molecules. They received their names because they require the presence of calcium ions to properly functions. There are many types of cadherins and each is expressed at a certain time in embryonic development.

When the sperm meets with an egg, the sperm un

Some common words found in the essay are:
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Approximate Word count = 1934
Approximate Pages = 8 (250 words per page double spaced)