For this question it is important to know the distinction between the genetic material being separated in meiosis I versus meiosis II. During meiosis I homologous chromosomes are separated, and during meiosis II sister chromatids are separated. Reduction of ploidy therefore occurs during telophase I, after the nuclear envelope reforms (due to the segregation of homologous chromosomes). During anaphase I there are technically still 46 chromosomes in the cell, even though each contains two sister chromatids and have been pulled to different regions of the cell. The total amount of genetic material has not changed.

Note that during anaphase of meiosis II ploidy is also at 46 chromosomes. At this point, sister chromatids have been separated from each of the 23 chromosomes present, resulting in 46 separate genetic units. The cell is still considered haploid, since the homologous chromosomes are not present.

Meiosis has many key differences from mitosis, despite the fact that both are used to divide a parent cell into daughter cells. One of the main differences is that meiosis involves the separation of homologous chromosomes, which halves the chromosome number in the daughter cells. This event does not take place in mitosis, because both daughter cells are are still diploid following division. 

Meiosis results in genetically unique daughter cells due to the event of crossing over and the phenomenon of independent assortment. Crossing over takes place during prophase I when the homologous chromosomes come in contact with each other to form tetrads.

During anaphase I, spindle fibers form and homologous chromosomes (each consisting of two chromatids) move toward opposite poles of the cell. During anaphase II, sister chromatids separate at their centromeres and are pulled toward opposite poles of the cell. Synapsis happens during prophase I, not anaphase I, as chromosomes condense and homologs align.

Gametes are the types of cells produced by meiosis. For example, a mature spermatozoan is a type of gamete. Adult cells are diploid, and through meiosis, gametes are haploid. Therefore, gametes have half of the genetic material of a mature cell. Thus, a gamete would be reflected with 0.5X as the amount of genetic material.

The mesoderm gives rise to the "middle" cells that line the body, such as the muscle. The notocord is a critical embryonic structure that forms from the mesoderm, and myoblasts and fibroblasts are cells that line the body. The alveoli are part of the lung, which originates from the endodermal germ layer.

Gastrulation is the process of cells from the epiblast ingressing into the embryo to form the three germ layers of bilaterally symetrical animals. When gastrulation is complete, the embryo is referred to as a gastrula. The group of cells that lead the migration of cells into the embryo are called the primitive streak, and they undergo epithelial to mesenchymal transition to be able to migrate. You can image the epithelium as many tightly packed squares. Becomimg mesenchymal turns these cells into more malleable stage, like a soft cushion. They can then "drop" into the inner layer of the blastocyst and lead the process of gastrulation.

Determination is the process of a cell committing to a particular fate and is influenced by the cell's environment as well as its own genome. It's not possible to tell the difference between an undetermined and a determined cell since determined cells do not change in appearance. After determination comes differentiation. Differentiation results from differential gene expression (transcription, mRNA splicing, and translation). 

The correct answer is gastrulation. Gastrulation occurs through five stages (1. invagination 2. involution 3. ingression 4. delamination 5. epiboly) and results in the formation of a gastrula with the mesoderm, endoderm, and ectoderm germ layers. Blastulation is the formation of the single germ layered blastula, which is a process that precedes gastrulation. Somiteogenesis forms somites in developing embryos to give rise to the future spine. Myogenesis is the formation of muscle tissue. Morphogenesis is the process of an organism forming its shape, driven by cell cycle progression, differentiation, and subsequent development of organs and appendages. 

Gastrulation is a process occurring in week three of development. At this time, the epiblast will further differentiate into three germ layers: the ectoderm, the mesoderm, and the endoderm. As a result, we can say that gastrulation allows for the creation of the mesoderm.