Implantation of the Ovum Within a few hours of fusion of the sperm and the ovum, cell division begins. The number of cells doubles with each successive division until a hollow ball of rapidly dividing cells is produced. Throughout the process the ovum is moving down the Fallopian tube toward the uterus. Five to six days after the fusion, the ovum reaches the uterus and implants itself in the soft wall. The hollow ball of cells now contains up to one hundred cells, arranged as an outer ring and an inner mass. The outer ring grows outward and penetrates the uterine tissue, while the inner mass divides into two separate layers, an inner endoderm and an outer ectoderm. These two layers become separated form the outer ring and by nine days form two cavities with the hollow ball. The ectoderm later spreads around the periphery of the smaller cavity, forming a sac, the amnion, which will surround and protect the developing embryo. The endoderm similarly encloses the larger cavity to form a yolk sac, which will have a nutritive function.

Three Cell-Producing Layers All the cells of the body are produced from the fertilized single-celled ovum. Although all the cells carry the same genetic information, different genes operate to produce each of the three cell types--ectoderm, mesoderm and endoderm---of which the entire human body is composed. The embryo has an ectodermal, mesodermal and endodermal layer, and each layer produces particular kinds of tissue. The endoderm, for example, produces the surface of the alimentary canal and associated glands, and surface of the lungs, trachea and larynx.. The Mesoderm produces tissue specific to bone, cartilage, muscle, the lining of blood vessels and the lining of the ureters. Ectoderm produces tissue for the brain, nerves, epidermis, hair, nails, the lining of the nose and anus, and the sensory organs.

Gastrulation is a developmental phenomenon by which a relatively limited number of blastodermal cells undergoes a stereotypical set of cellular movements that leads to formation of the three germ layers: ectoderm, mesoderm and endoderm. Production: Maryam Razmpoosh, Alexande Oligny, Luc Oligny; CHU Sainte-Justine, Université de Montréal, QC.

From Embryo to Fetus-Part One Twenty days after fertilization the embryo is a three-layered disk of ectoderm, mesoderm and endoderm cells. Eight days later the yolk sac has shrunk, the amnion has enlarged and the embryo has recognizable features. The embryo at three weeks, has a primitive heart and rudimentary brain tissue. One week later the heart is pumping blood around the body and into the placenta. The embryo at five weeks has twenty-eight pairs of tissue blocks, which later form bones and muscles. Limb buds and rudimentary eyes are visible and the tail is receding. One week later the hands have rudimentary fingers, the eyes have lenses and the liver and heart are visible. At seven weeks the head grows rapidly, external ears are visible, the eyes are open and the tail has disappeared. At eight weeks the embryo is now called the fetus. It has most of its tissues and organs and well-developed hands and feet.

Process of Gastrulation after Implantation of Embryo

This planarium was found in my front yard on 5/01/08. I live in South Arkansas (US). I have found 3 others in the yard when I first moved into this house 3 years ago. I spent months on the net till I finally found out what it was. Planaria sp., or flatworm, is a common freshwater, non-parasitic member of the phylum Platyhelminthes, class Turbellaria. It moves by beating cilia on the ventral dermis, allowing it to glide along on a film of mucus. Some move by undulations of the whole body by the contractions of muscles built into the body wall. They exhibit an extraordinary ability to regenerate lost body parts. For example, a Planaria split length wise or crosswise will regenerate into 2 separate individuals. The size ranges from 3 to 12mm, and the body has two eye-spots (also known as ocelli) that can detect the intensity of light. The eye-spots act as photoreceptors and are used to move away from light sources. Planaria have 3 germ layers (ectoderm, mesoderm, and endoderm), but lack a body cavity or coelom. They have a single-opening digestive tract.