Problem 43

Background and context

After fertilization of male and female gametes, a single-celled zygote is formed. The zygote undergoes rapid cell division many times (cleavage) to form an embryo of about 100 cells. The embryo, at this stage, is called a blastula. The blastula cells arrange themselves in a spherical layer of cells called the blastoderm and a yolk-filled center called the blastocoel. There are two ways by which cleavage occurs: holoblastic and meroblastic. Holoblastic is total cleavage and occurs in placental mammals, including humans. Meroblastic cleavage is partial cleavage and is found to occur in birds and reptiles.