Development of Human, Chick and Frog
The egg of the amphibian is fertilized externally and contains a large amount of yolk. After the egg is fertilized there is a reorganization of the cytoplasm. When the sperm penetrates the egg the pigment cap rotates toward the point of sperm penetration and a gray crescent appears on the side of the egg opposite the point of the sperm entry. Since the yolk is present in larger amounts the egg divides unevenly forming larger cells in the vegetal hemisphere. The cleavage forms a blastula but the amphibian blastocoel is small and off center. The blastopore appears as a crescent shaped slit between the gray crescent and the vegetal hemisphere. Gastrulation (the process of inward movement) in amphibians is an extensive series of coordinated cellular movements; migration of single cells and sheets of cells, and the folding of cell layers. The initiation of Gastrulation starts when the blastopore is formed. The cells at the dorsal lip of the blastopore change shape and move t!o the interior and are replaced by cells that move to the surface. The cells move over the lip. When the cells get into the embryo they move away from the blastopore deeper inside. The direction they move will become the anterior-poste
------------------------------------------------------------------------ l materials entering and leaving the placenta. The placenta is formed from the maternal tissues of the endometrium which make contact with the chorionic tissues of the embryo. Although the blood of the mother and fetus never mix diffusion occurs through the placenta of materials. By the end of the 3rd month the placenta secrete its own estrogen and progesterone and at this point the corpus luteum has degenerated. In the second week the body axis's develops. In the third week the central nervous system begins to form. By the end of the 1st month the embryo is c shaped. It has 40 pairs of somites which muscle bone and connective tissue form. The germ cells are set aside in the embryo by the 3rd week of pregnancy and complete their migration to the gonads by the 38th day. During the second month the embryo is referred to as a fetus. Arms, legs, elbows, knees fingers and toes are forming. The gallbladder and pancreas are present and the digestive tract is differentiated.! e lens of the eye is secondary induction. The somites on the sides of the notochord differentiate into sclerotome cellsskeletal elements, dermatome cellsskin and myotome cellsmuscles. The lateral plate forms the kidneys, gonads and ducts of the excretory and reproductive systems. The coelom forms the lining of the thoracic and abdominal cavity and the outer layer of internal organs. The endoderm differentiates into the respiratory tracts and related organs. Portions of the endodermal pouches develop into auditory tubes, tonsils, parathyroid and thymus gland. Posterior to this the lungs develop. More posterior to this the gut begins to differentiate into liver, gallbladder and pancreas. These are all made up of tissue from the mesoderm and ectoderm. Morphogensis then happens. This is where the body gains character and shape. Cell growth inc/dec, changes in adhesion to neighboring cells, deposition of extracellular material and changes in shape! rm and mesoderm. Its first function is excretion and then later on envelops the embryo. It obliterates the extraembryonic coelom and its walls fuse with the chorion. It is called the chorioallantoic membrane and it acts as an efficient respiratory membrane for the embryo. The f
Some common words found in the essay are:
AP-Biology Essay, neural tube, cells form, amount yolk, neural plate, neural ectoderm, lateral plate, contains amount yolk, lateral plate mesoderm, arms legs, plate mesoderm, chordamesoderm cells, move interior,
Approximate Word count = 1565
Approximate Pages = 6 (250 words per page double spaced)
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