The zygote will divide into a morula once eight or more cells are created. The morula is still characterized by cellular cleavage, and does not increase beyond the size of the original zygote cell. Eventually, the morula develops into a blastula, characterized by the introduction of the G2 phase in the cell cycle. This allows for growth and development of the cells before subsequent divisions. The blastula develops a fluid-filled cavity known as the blastoceole, and is given the name blastocyst. The blastocyst will then grow into a gastrula after gastrulation, and later a neurula (after neurulation). 

After fertilization occurs, the zygote undergoes a series of cellular divisions in a process called cleavage. This is followed by the formation of the blastula, a hollow sphere of cells. Gastrulation occurs next, where the embryo is divided into three germ layers: the endoderm, mesoderm, and ectoderm (from inside to outside). Hence, gastrulation is the stage in which this process first occurs. Neurulation and organogenesis occur after gastrulation.

Cortical reaction occurs in fertilization when a sperm enters the cytoplasm of an oocyte. The reaction causes a change in the zona pellucida that prevents polyspermy. Determination refers to the point at which a cell becomes committed to a certain developmental path during embryogenesis, and differentiation refers to the process by which that cell actually becomes a specialized tissue cell. Determination occurs very early during embryogenesis, whereas differentiation occurs later in the developmental process.

After the third cleavage, the embryo would be composed of 8 cells. If every cleavage step up up to this point was indeterminate, then each cell still has the necessary components to develop into a complete organism. The result would be 8 genetical identical organisms.

The vegetal pole and the blastopore are not considered the primary layers formed early in gastrulation, nor will they give rise to skin and nervous system structures. The endoderm will ultimately form the digestive organs and lungs, and the mesoderm will form the muscles, kidneys, and bones. The correct answer is ectoderm, which will form skin and nervous system tissues. 

The three primary germ layers are the ectoderm, mesoderm, and endoderm. There are some general structures that can be attributed to the germ layers during development. The ectoderm gives rise to the outer coverings of the body, such as the skin, and to the central nervous system. The ectoderm produces the notochord and primitive neural structures very early in development. The mesoderm gives rise to the muscles and bones. Finally, the endoderm gives rise to the digestive tract and the liver.

The primitive streak forms in the blastula stage and establishes symmetry (left-right and cranial-caudal body axes). This spatial differentiation determines the site of gastrulation and initiates formation of the three germ layers. The epiblast (precursor to the ectoderm) invaginates to form the primitive streak. Cells from the primitive streak give rise to the mesoderm and the endoderm. Formation of the primitive streak marks the beginning of gastrulation.

The blastula, or blastocyst, is made up of blastomere cells and a fluid-filled cavity called the blastocoel. 

There are two regions of the blastocyst: the inner cell mass and the trophoblast. The inner cell mass gives rise to the primitive endoderm and the epiblast, which later gives rise to the three germ layers during gastrulation. The trophoblast is the layer of cells forming the outer ring of the blastocyst. It secretes factors to make the blastocoel and is kept separate from the inner cell mass. All fetal structures eventually develop from the inner cell mass, while the trophoblast helps maintain the fetal environment and placenta.

Once a committed cell begins to develop specialized functions, it is known as differentiation. Before a cell differentiates, it makes a commitment to a certain cell type, first by specification, which is reversible, and then by determination, which is irreversible. Once a cell is committed to a cell type, it undergoes differentiation to develop specific cell characteristics.

Induction is a process in which cells induce adjacent cells to commit to a certain cell type. 

The correct answer is master transcription factors. The type of master transcription factor expressed in a cell depends on the ultimate cell type it will become. Master transcription factors have higher affinity for cell identity genes. Each cell type has a different profile of master transcription factors that are reliably expressed. Mediator facilitates binding and recuitment of many transcription factors. Chromatin remodelers change the epigenetic state in a cell, and pioneering transcription factors are the first factors to bind DNA, even in heterochromatin regions.