The gastrula is formed during the second week following fertilization. At this stage, a process called gastrulation takes place. During gastrulation, the three primary germ layers are formed: the ectoderm, the mesoderm, and the endoderm.

Prior to gastrulation, the embryo is a blastocyst, and prior to that it is a morula. The morula forms soon after fertilization and is classified by cleavage divisions, increasing the number of cells without increasing the size of the embryo. The blastocyst is characterized by the formation of the inner cell mass and trophoblast; implantation occurs during this stage. After implantation, gastrulation occurs. After the gastrula stage, the embryo begins the process of neurulation (development of the primitive streak and notochord) and becomes a neurula.

The correct order is listed below.

1. Fertilization: the joining of an egg and sperm

2. Cleavage: early cell division in an embryo before a specific structure is formed

3. Blastula Formation: the cells of the embyro form a hollow structure filled with fluid

4. Gastrulation: the cells in the embryo migrate to form the three germ layers; the hole with fluid formed during the blastula stage is now filled with cells

Gastrulation is the phase in embryogenesis in which the single-layered blastula is reorganized into a trilaminar structure called the gastrula. These three germ layers are called the endoderm, mesoderm, and ectoderm and give rise to individual organs during organogenesis.

The blastula is implanted into the uterine lining and the morula undergoes rapid cell divisions (cleavage) after fertilization of the zygote.

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.

Fertilization is the complete fusion of egg and sperm. If the chemically-altered sperm are unable to fertilize an egg, there will be no resulting pregnancy. Oogenesis is the process of ovum production. Spermatogenesis is not correct because the chemical is altering the protein coating after production, as the birth control is taken by the woman.

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 series of mitotic divisions that convert a zygote to a multicellular blastocyst is known as cell cleavage. All of the listed answers are possible as a result of cleavage mishaps. Tetraploidy can result from cells that fail to cleave at the end of mitosis. Monozygotic twins are a result of unexpected cleavage events where the zygote splits into two identical halves. Depending on the timing of a cleavage event in a zygote, the split can result in conjoined twins. Mosaicism is a result of improper chromosome segregation during cleavage that can result two distinct karyotypes in a single embryo.

Embryonic cleavage is the division of cells without cell growth. Once an ovum is fertilized by a sperm cell, it is called a zygote. The zygote undergoes multiple rapid cell cycles (rounds of mitosis) without significant growth, producing a dense cluster of cells that is the same size as the original zygote. This process is called cleavage.

Embryogenesis is the process by which an embryo forms, and begins with fertilization of an ovum by a sperm cell. Once an ovum is fertilized, it is referred to as a zygote. The zygote undergoes rapid mitotic divisions without any significant cell growth, a process called cleavage. After four cell divisions occur, a ball of sixteen cells is produced, which is called the morula. After the seventh cleavage, a ball of 128 cells called the blastula is produced. The blastula allows the addition of the G1 and G2 phases into the cell cycle, permitting the cells to grow in size before dividing. The blastula develops an inner cell mass and trophoblast, and becomes implanted in the uterine lining. A gastrula is formed when cells migrate to the interior of the blastula and form three germ layers: the endoderm, mesoderm, and ectoderm. A neurula forms after differentiation of the ectoderm forms the primitive central nervous structures.