Summary
The early stages of embryonic development begin with fertilization. The process of fertilization is tightly controlled to ensure that only one sperm fuses with one egg. After fertilization, the zygote undergoes cleavage to form the blastula. The blastula, which in some species is a hollow ball of cells, undergoes a process called gastrulation, in which the three germ layers form. The ectoderm gives rise to the nervous system and the epidermal skin cells, the mesoderm gives rise to the muscle cells and connective tissue in the body, and the endoderm gives rise to columnar cells and internal organs.
Glossary
acrosomal reaction
series of biochemical reactions that the sperm uses to break through the zona pellucida
blastocyst
structure formed when cells in the mammalian blastula separate into an inner and outer layer
gastrulation
process in which the blastula folds over itself to form the three germ layers
holoblastic
complete cleavage; takes place in cells with a small amount of yolk
inner cell mass
inner layer of cells in the blastocyst
meroblastic
partial cleavage; takes place in cells with a large amount of yolk
polyspermy
condition in which one egg is fertilized by multiple sperm
trophoblast
outer layer of cells in the blastocyst
zona pellucida
protective layer of glycoproteins on the mammalian egg