Embryonic Development Perfect Paragraph

Development of a human embryo comes about in three stages: blastulation, gastrulation, and neurulation. First, the egg and sperm fuse into a zygote composed of an internal plasma membrane and an external zona pellucida. The zygote develops into a morula by cleaving internally, dividing into multiple cells without gaining volume. The cells inside the zona pellucida start compacting and differentiation begins. A mass of embryoblasts develops at the centre of the zona pellucida surrounded by a layer of trophoblasts. The embryoblasts migrate to the top half of the cell, leaving behind an empty space known as the blastocoel. This stage marks the end of blastulation at which point the embryo is known as a blastocyst. Then, the embryoblast mass develops an inner cavity called the amniotic cavity. The layer of cells directly under the amniotic cavity differentiate into epiblasts and the layer of cells under the epiblasts become hypoblasts. Collectively, these two cell layers form the bilaminar disc. Next, a group of epiblasts known as the primitive streak form along the middle of the epiblast layer. These cells will migrate downwards past the epiblast layer to form a third layer between the epiblasts and hypoblasts. The three layers of this trilaminar disc are now known as germ layers. The topmost layer is the ectoderm, the middle layer is the mesoderm, and the bottom layer is the endoderm. The ectoderm will form the nervous system and the skin; the mesoderm will form the bones and muscles; and the endoderm will form the viscera. The creation of the three germ layers is known as gastrulation.