Development+homework+EAT

Human Life Cycle 16.1 p. 320 (1) How many chromosomes does a mother contribute to a new individual? A father?
 * A mother contributes 23 chromosomes. The egg contains 23 chromosomes within and makes the half of the 46 chromosomes needed.
 * A father contributes 23 chromosomes. The sperm has 23 chromosomes within to add to the 46 chromosomes to make a person.



(2) Where does meiosis occur in males? in females? (3) Compare and egg cell to a sperm cell. (4) What is the difference between meiosis and mitosis.
 * (Males) Meiosis takes places in the testes while sperm is being made.
 * (Females) Meiosis takes place in the ovaries while the eggs are being made.
 * Sperm is smaller than the egg. The egg is made to stay stationary while the sperm is made to swim towards the egg to fertilize. The egg only carries chromosomes while the egg has chromosomes and cytoplasm.
 * Meiosis is reduction division. This only happens in the males testes during production of sperm.... and in the ovaries when eggs are being made. The chromosomal number is reduced from 46 chromosomes to 23 chromosomes. It requires two cycles (Meiosis 1 and meiosis 2)




 * Mitosis is duplication division. The cell forming makes an exact replica of itself. This takes place when the body is trying to repair tissues and help growth of tissue.



Development Chapter 17 17.1 (5) Describe in detail how fertilization occurs
 * Sperm has a tail (flagellum) which enables it to swim towards the egg. The middle of the sperm has mitochondria while the head has the nucleus. The head of the sperm "fuzes" with nucleus of the egg. The egg is surrounded by several layers, which act as a nourishing cushion while the egg is in the ovary. Several sperm penetrate the corona radiata, and attempts to penetrate the zona pellucida. Only one sperm enters the egg. Once the sperm head makes a tight bond with the zona pellucida, the acrosome release digestive enzymes that make a pathway for the sperm. Once the sperm connects to the egg their plasma membranes connect and the sperm officially enters the egg. The Nucleuses of the two then connect. Only sperm should enter the egg (not visa versa.)



(6) What is polyspermy and how is prevented during fertilization?
 * Polyspermy is when an egg as been fertilized by more than one egg. The person would have three or more copies of each chromosomes. Prevention occurs when the sperm enters the egg, the plasma membrane of the egg depolarizes, which prevents the binding of other sperm. Also Enzymes are released causing the zona pellucida unable to be fertilized.

17.2 (7) Describe the processes involved in development. (8) What are the extraembryonic membranes and what is their role?
 * Cleavage happens right after fertilization. This is when the zygote starts to divide. This division is mitotic, and each cells has a full complement of chromosomes and genes.
 * Growth occurs during embryonic development. Cell division along with increased daughter cells occurs during this stage.
 * Morphogenesis has to do with the shaping of the embryo and is first noticeable when certain cells move in relation to the other cells. These movements make the embryo assume different shapes.
 * Differentiation this is when the cells have one specific structure and function. The first system to differentiate is the nervous system
 * Chorion develops the fetal half of the placenta (the organ that has the embryo.) It provides the fetus with nourishment and oxygen takes away the waste.
 * Allantios accumulates small amounts of urine that is produced by the fekal kidneys and then later turns into the urinary bladder.
 * Yolk sac is the first embryonic membrane that appears. The sac contains little yolk in humans, but has a lot of blood vessels.
 * Amnion has fluid to cushion the embryo that develops into a fetus.



(9) What happens during pre-emrbyonic development? (10) What happens during embryonic development?
 * Pre-embryonic development happens in the first week of development. The zygote divides repeatedly while it passes the oviduct to the uterus. The blastocyst's arrange themselves so there is an "inner cell mass" which is surrounded by an outer layer of cells. The inside will become the embryo and the outside layer will become the chorion. Sometimes during development the cells separate and crate two pre-embryos which will later become twins.
 * It starts during the second week and lasts through the final week of the second month of development.
 * Second week
 * the embryo starts implanting itself in the wall of the uterus (implantation.) Once this is successful the person is considered pregnant. During this time the chorion releases enzymes that get rid of the tissue and blood vessels of the endometrium of the uterus. It releases human chorionic gonadotropin (the hormone that is what pregnancy tests are based off of.) The inner cell mass turns into the embryonic disk and then two more extraembryonic membranes form. The yolk sac is the first place for blood cells to form. The amniotic fluid is an insulator for heat and cold, it absorbs shock (the mother moving or exercising... this process is known as gastrulation.) The three primary germ layers are formed.
 * Third week
 * This is when the nervous system and heart begin to form. The nervous system appears as a thickening along the wall of the embryo. Invagination occurs as neural folds appear. Once these folds come together they make a tube which will become the brain and spinal cord. The development of the heart starts in the third week and lasts for a week. To start there are right and left hear tubes. When these two tubes come together the heart is able to start pumping blood.
 * Fourth week
 * There is a bloody stalk that connects the embryo to the chorion (which has chorionic villi.) The allantois is in the stalk and the blood vessels of this stalk will become the umbilical blood vessels which sill form the umbilical cord. Limb buds appear, which will become the arms and the legs (even hands and feet will develop from this.) The head will become larger and the sense organs become stronger. It is even possible to see the ears and eyes at this stage.
 * Sixth- eight weeks
 * The embryo becomes recognizable as a human. There is brain development, which allows the head to have an appropriate relationship with the body while the neck is developing. The nervous system is developed enough to have reflex responses. All of the organ systems have been developed (by the end of this period.)



(11) What are the primary germ layers and what body structures come from each layer?
 * Ectoderm is the beginning of the outer tissue that covers the body. It comes from the outermost germ layer. The nervous system forms from the ectoderm, as well as the outer part of the integument.
 * Mesoderm forms during the gastrulation, when the move inward from the endoderm to make another layer between endoderm and ectoderm. It is made of the skeleton, muscular system, urinary system, and the dermis of the skin.
 * Endoderm is formed during embryogenesis. It is formed when cells move in along side the archenteron. It makes up the lining for the digestive tract.[[image:vertebrate_asymm_genes.jpg]]