The zygote becomes a morula or solid ball of cells as the cells of the sperm/egg fusion grows then divides, grows again, then divides. This then becomes a blastocyst (mammals) and the blastocyst is what is implanted in the endometrium.4 Cleavage is mitotic divisions without cell growth and it produces the morula. Cleavage produces the morula and the morula hollows out into the blastocyst. Over the period of nine months, the blastocyte undergoes organogenesis and gastrulation. At birth, the baby switches from getting oxygen from mother’s blood to breathing on its own. The bay also switches from getting nutrients from mother’s blood to suckling on the breast. Fetal circulation (which bypasses lungs and liver) transitions to normal circulation by the closing off of ducts and openings. Cells from the surface migrate inwards. In mammals, the cells start migrating inward at the primitive streak and forms the primary germ layers (endoderm, mesoderm, and ectoderm). The cells that migrate inwards form the endoderm, the cells that remain outside become the ectoderm and the cells in the middle become the mesoderm. Ectoderm forms the brain and spinal cord and the ectoderm does so by folding into a tube. The major structures arising out of the endoderm are the innermost layers such as the gut, lungs, and digestive internal organs. The major structures arising out of the mesoderm are middle layers such as muscle, blood and bone tissues, and internal organs. The major structures arising out of the ectoderm are the outermost layers such as the skin and nerves. During apoptosis, strong proteases are activated and they digest the cell from within. In mammals, the proteases are called caspases. The spaces between our fingers are created by apoptosis. In the third week of embryonic development, the three primary cell types begin their development into the tissues and organs of the body.4 This stage in development is called neurulation. The notochord is a flexible rod in vertebrates and is the first characteristic vertebrate feature to form. It forms along the midline of the embryo as soon as gastrulation is complete and is found below its dorsal surface. The neural tube is the second characteristic vertebrate feature and it forms above the notochord. It will later differentiate into the spinal cord as well as the brain. Two strips of cells typically break away just before the neural tube closes, and these form the neural crest. Neural structures found in the vertebrate body arise from these neural crest cells.




1) Maton A, Hopkins JJ, LaHart S, Quon Warner D, Wright M, Jill D (1997). Cells: Building Blocks of Life. New Jersey: Prentice Hall. pp. 70–4

2) Wilbur, Beth, editor. The World of the Cell, Becker, W.M., et al., 7th ed. San Francisco, CA; 2009.

3) Kent, M. (2000). Advanced Biology. Pages 246 -247. Oxford University Press.

4)  M.B.V. Roberts, J. M. (1985) Biology for CXC. Pages 266 – 267. Cheltenham: Thomas Nelson and Sons Limited.

5)  Slack, J.M.W. (2013) Essential Developmental Biology. Wiley-Blackwell, Oxford.

6) Hayflick L; Moorhead PS (December 1961). “The serial cultivation of human diploid cell strains”. Exp. Cell Res. 25: 585–621.


  • Human_embryogenesis
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