Biology » The Reproductive System » Fertilization and Early Embryonic Development

Summarizing Fertilization and Early Embryonic Development


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.


acrosomal reaction

series of biochemical reactions that the sperm uses to break through the zona pellucida


structure formed when cells in the mammalian blastula separate into an inner and outer layer


process in which the blastula folds over itself to form the three germ layers


complete cleavage; takes place in cells with a small amount of yolk

inner cell mass

inner layer of cells in the blastocyst


partial cleavage; takes place in cells with a large amount of yolk


condition in which one egg is fertilized by multiple sperm


outer layer of cells in the blastocyst

zona pellucida

protective layer of glycoproteins on the mammalian egg

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