The early stages of embryonic development, such as fertilization, cleavage, blastula formation, gastrulation, and neurulation, are crucial for ensuring the fitness of the organism.

Fertilization is the process in which gametes (an egg and sperm) fuse to form a zygote. The egg and sperm each contain one set of chromosomes. To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. The acrosomal reaction causes an egg to prevent additional sperm from penetrating. As the egg completes meiosis II, sperm and egg nuclei fuse. 

The formed zygote undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage that produces over 100 cells in the embryo. This process is called the blastula formation. During cleavage, the cells divide without an increase in mass; that is, one large single-celled zygote divides into multiple smaller cells. As the blastula forms the blastocyst in the next stage of development, the cells in the blastula arrange themselves into the inner cell mass, and an outer layer.

The typical blastula is a ball of cells. The next stage in embryonic development is the first cell movement and the formation of the primary germ layers. The cells in the blastula rearrange themselves spatially to form three layers of cells. This process is called gastrulation. These three germ layers are the endoderm, the ectoderm, and the mesoderm. The ectoderm gives rise to skin and the nervous system; the endoderm to the intestinal organs; and the mesoderm to the rest of the organs.

Following gastrulation, the neurulation process develops the neural tube in the ectoderm, above the notochord of the mesoderm. The ectoderm gives rise to the nervous system by folding into a neural tube.

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Key Points

• Upon fusion of the two plasma membranes, the sperm’s nucleus enters the egg and fuses with the nucleus of the egg.

• Both the sperm and the egg each contain one half the normal number of chromosomes, so when they fuse, the resulting zygote is a diploid organism with a complete set of chromosomes.

• Gastrulation takes place after cleavage and the formation of the blastula.

• The ectoderm gives rise to skin and the nervous system; the endoderm to the intestinal organs; and the mesoderm to the rest of the organs.

• Neurulation is the formation of the neural tube from the ectoderm, which forms into a neural tube.

Key Terms

blastula: a 6-32-celled hollow structure that is formed after a zygote undergoes cell division

inner cell mass: a mass of cells within a primordial embryo that will eventually develop into the distinct form of a fetus in most eutherian mammals

gastrulation: the stage of embryo development at which a gastrula is formed from the blastula by the inward migration of cells

neurulation: The process that forms the vertebrate nervous system in embryos.

notochord: Composed of cells derived from the mesoderm, this provides signals to the surrounding tissue during development.