Fertilization is the fusion of a sperm and an egg, a process that initiates genetic variation. In humans, both the sperm and the egg contribute 23 chromosomes each. Their combination results in the formation of a zygote, a single cell containing a complete set of 46 chromosomes, organized into 23 pairs.

Fertilization

Gastrulation occurs in embryogenesis after cleavage.  The blastomeres that make up the blastula extensively rearrange themselves to contain three germ layers: the endoderm, the mesoderm, and the ectoderm.  These three layers will give rise to all of the organs of the body.  After gastrulation, the embryo is said to be a gastrula.  

Gastrulation

The process of developing tissues and organs.  This results from the cells of the three germ layers interacting and rearranging.  

Organogenesis

The larval stages of development come after organogenesis in embryogenesis.  This is the sexually immature stage of the organism.  

Larval Stage

Gametogenesis

Embryonic cleavage is the stage of embryogenesis immediately following fertilization, characterized by rapid mitotic cell divisions. During this process, the zygote's cytoplasm is partitioned into many smaller cells that eventually arrange themselves into a spherical shape.

Embryonic Cleavage

The stages of development between fertilization and hatching.  These stages include:

- fertilization
- cleavage
- gastrulation
- organogenesis
- larval stages
- maturity
- gametogenesis

Michigan State University

A developing organism before it hatches or is born.  

Embryo

Barresi, M. J., &amp; Gilbert, S. F. (2020). Developmental Biology (12th ed.). New York: Sinauer.

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