ANATOMY

Detailed explanation-1: -The process of embryogenesis begins when an egg or ovum is fertilized by a sperm cell to form a zygote. This zygote then undergoes mitotic division, a process that does not result in any significant growth but creates a multicellular cluster called a blastula. This process is referred to as cleavage.

Detailed explanation-2: -After fertilization, the development of a multicellular organism proceeds by a process called cleavage, a series of mitotic divisions whereby the enormous volume of egg cytoplasm is divided into numerous smaller, nucleated cells. These cleavage-stage cells are called blastomeres.

Detailed explanation-3: -A zygote undergoes rapid cell divisions (cleavage) to form a spherical ball of cells: the blastula; this will further develop into a blastocyst.

Detailed explanation-4: -After the blastula develops, it undergoes transition to the gastrula (q.v.), a process called gastrulation. In organisms such as mammals, the earlier morula (q.v.), a berrylike cluster of cells, develops into a somewhat different form of blastula, the blastocyst (q.v.).

Detailed explanation-5: -The blastula is formed in the second stage of embryonic development. This happens after fertilization, once the zygote begins rapid cellular division.