![]() ![]() The duration of this period varies between species.įor most species, the amount a fetus grows before birth determines the length of the gestation period. See also: List of mammalian gestation durationsįor mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The flow of fluids throughout the embryo begins at this stage. Endocardial cells begin to form the myocardium.Īt about 24 days past fertilization, there is a primitive S-shaped tubule heart which begins beating. Blood cells begin to form on the sac in the center of the embryo, as well as cells which begin to differentiate into blood vessels. Secondary blood vessels also begin to develop around the placenta, to supply it with more nutrients. Blood cells are already being produced and are flowing through these developing networks. The blood system continues to grow networks which allow the blood to flow around the embryo. It begins growing in a concave area known as the neural groove. The embryo's nervous system is one of the first organic systems to grow. ![]() It is shaped like a pear, where the head region is larger than the tail. The ectoderm forms the skin, nails, hair, cornea, lining of the internal and external ear, nose, sinuses, mouth, anus, teeth, pituitary gland, mammary glands, eyes, and all parts of the nervous system.Īpproximately 18 days after fertilization, the embryo has divided to form much of the tissue it will need. It also gives rise to the spleen, and produces blood cells. The mesoderm forms muscle, bone, and lymph tissue, as well as the interior of the lungs, heart, and reproductive and excretory systems. The endoderm later forms the lining of the tongue, digestive tract, lungs, bladder and several glands. These three layers give rise to all the various types of tissue in the body. ![]() The ectoderm begins to grow rapidly as a result of chemicals being produced by the mesoderm. The narrow line of cells begin to form the endoderm and mesoderm. Its growth makes the embryo undergo gastrulation, in which the three primary tissue layers of the fetus, the ectoderm, mesoderm, and endoderm, develop. This later develops into the umbilical cord.įollowing this, a narrow line of cells appears on the surface on the embryo. For the next 24 hours, connective tissue develops between the developing placenta and the growing embryo. The small sac inside the blastocyst begins producing red blood cells. A system of blood and blood vessels now develops at the point of the newly forming placenta, growing near the implantation site. Several days later, chorionic villi in the forming placenta anchor the implantation site to the uterus. At the same time, the bottom layer forms a small sac (if the cells begin developing in an abnormal position, an ectopic gestation may also occur at this point). The top layer becomes the embryo, and cells from there occupy the amniotic cavity. The inner cell mass of the blastocyst divides rapidly, forming two layers. This is the first stage in the growth of the placenta. The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood, which in turn stimulate the production of capillaries. The cells on the exterior of the blastocyst begin excreting an enzyme which erodes epithelial uterine lining and creates a site for implantation. In 24 to 48 hours, the zona pellucida breaches. The presence of the blastocyst means that two types of cells are forming, an inner-cell mass growing on the interior of the blastocele and cells growing on the exterior of it. This new structure with a cavity in the center and the developing cells around it is known as a blastocyst. Cells grow increasingly smaller to fit in. Then, a flat layer cell forms on the exterior of this cavity, and the zona pellucida, the blastocyst's barrier, remains the same size as before. 14 fetuses showing stages of embryo development in the African elephant After implantation Ī blastocele is a small cavity on the center of the embryo, and the developing embryonary cells will grow around it. ![]()
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