Notes

The sperm and egg are also called gametes
Animal sperm cells are usually very small and have a flagellum that they use to swim toward and egg
The cytoplasm of the larger egg cells contains yolk, an energy-rich mass if nutrients
Each gamete nucleus has one-half of the chromosome set found in each of the parent's cells
A fertilized egg, or zygote, is the earliest stage of the embryo
Before fertilization, the eggs of many animals are metabolically inactive
When the zygote begins to divide, it is as metabolically active as most adult cells.
Activation has two other major effects:
1 a rapid change in the plasma membrane, which blocks fertilization by a second sperm
2 a rearrangement of the zygote cytoplasm by movements in the cytoskeleton that helps produce differences among cells when they divide
Animal development includes growth, cell specialization, and formation of tissues and organs
Each type of cell that differentiates during development has a unique structure and function
Skin cells are touch, thin, flat cells that are specialized to protect the body
Nerve cells have long, thin branches that are specialized to transmit information
The mature human red blood cell lacks a nucleus and is specialized to transport oxygen
Proteins are the keys to differentiation in animal cells
Differences between cells in gene expression lead to differences in cell form and function
After fertilization, the zygote divided into two cells
But the end of cleavage, the embryo consists of a mass of many cells called a blastula
The cells are usually all of the same general size and appearance
Outer layer -ectoderm
Middle layer-mesoderm
Inner layer-endoderm