A blastula is the next step of development after a zygote.  It is formed in the cleavage process as blastomeres arrange into a sphere.  

Blastula

A zygote is formed through the process of fertilization.  It is a single cell that goes on to mitotically divide in order to produce all of the cells of an organism's body.  

Michigan State University

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

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

Developmental Biology (12th ed.)

Zygote

A zygote is the single cell created when a sperm and an egg fuse. Because the egg and the sperm each carry 23 chromosomes, their fusion results in a zygote with a total of 46 chromosomes, arranged in 23 pairs. Through this process, each parent contributes half of the offspring's genetic information.

A new single-celled organism is formed when a sperm and an egg fuse, with each contributing a set of chromosomes. If an error occurs causing one of the contributing cells to have an extra chromosome, what is the most direct consequence for the newly formed organism?

This microscopic image captures the moment of fertilization, showing a single sperm cell penetrating and fusing with an ovum (egg cell).

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