During spermatogenesis, one spermatogonia will become 4 separate functioning spermatozoa. In oogenesis, a primary oocyte will become 1 functioning ovum (or egg) while 3 polar bodies are also produced, which are nonfunctioning and never become eggs.

When a female ovulates, the oocyte is in metaphase II, until it becomes fertilized by a sperm. It then completes the second phase of meiosis to make a mature ovum. Remember that, unlike spermatogenesis, oogenesis begins during development. All oocytes undergo meiosis I during fetal development, and are kept suspended in prophase II until ovulation, when they shift to metaphase II. Meiosis only continues beyond metapahse II if fertilization occurs.

Oogenisis halts in the female embryo while the oocytes are trapped in meiosis II. At the beginning of the menstrual cycle a number of oocytes in primordial follicles are stimulated by pituitary gonadotropins to continue maturation.

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.