The term “zygote” is sued to describe a fertilized egg. The zygote is diploid and formed by the fusion of the sperm and egg gametes. The developing organism is called the zygote until it is composed of 16 cells during cleavage, when it is called the “morula.”

Developing embryos exhibit an asymmetry; the zygote can be described as having two hemispheres. These two poles are called the animal and vegetal poles. The vegetal pole contains more yolk and will become the endoderm. The animal pole contains less yolk and will become the ectoderm and mesoderm.

Holoblastic cleavage is the type of cleavage found in developing embryos with a low yolk concentration. This allows for the complete cleavage that forms planes that extend through the entirety of the zygote. Different cleavage patterns in holoblastic cleavage include bilateral, radial, rotational, and spiral. Animals whose zygotes undergo holoblastic cleavage include placental mammals, amphibians, and annelids.

Meroblastic cleavage is partial cleavage in zygotes with a high concentration of yolk. In developing embryos that undergo this process, cell divisions do not penetrate the yolk. Two types of meroblastic cleavage are discoidal and superficial cleavage. This process occurs in insects, some fish, and some types of birds.

Indeterminate cleavage takes place in most deuterostomes. It is a type of cleavage in which embryonic cells can be separated and successfully develop into different organisms.

Developing mammalian embryos exhibit rotational cleavage. In this type of cleavage, the first cell division occurs on meridionally. The next cell division in the two resulting daughter cells occurs meridionally in one and equatorially in the other.

In the developing embryo, transcription initiates at the end of cleavage, during a stage called the midblastula transition. 

The trophoblast is the outer layer of cells that make up the blastocyst. These cells provide nutrients to the developing embryo, and eventually develop into the placenta. During the later stages of cleavage, the trophoblast becomes distinguished from the inner layer of cells, which will develop into the embryo. There is a layer of fluid between the trophoblast and inner cell mass.

In a human egg cell the yolk—ooplasm—located at the cell center in an area containing the nucleus and nucleolus. The ooplasm is composed of fatty granules that support embryo development through the early stages of embryogenesis. In human oocytes, there is a relatively small amount of ooplasm compared to the embryos of other species.

During fertilization and throughout the process of cleavage, the developing embryo is located in the fallopian tube. Towards the end of cleavage, around day 5, the developing embryo enters the uterus, where it will implant in the uterine wall after blastulation.