GESTATION AND HUMAN DEVELOPMENT   A new individual is produced when a sperm fertilizes an egg, resulting in the formation of a zygote.  During the nine-month period of Gestation, a series of changes transform a single cell into a complex organism made of trillion of cells – a human.   OBJECTIVES:  Describe the processes of Fertilization, Cleavage, and Implantation.  Summarize the changes that take place during the development of an embryo.  List three stages of pregnancy and describe each stage.   FERTILIZATION   1. IN ORDER FOR AN OVUM TO BECOME FERTILIZED, SPERM MUST BE PRESENT IN THE FEMALE REPRODUCTIVE TRACT- MORE SPECIFICALLY, IN A FALLOPIAN TUBE.   2. During Ejaculation, 150 MILLION to 350 Million Sperm are deposited just a few inches from the Fallopian Tubes, in which Fertilization CAN OCCUR.   3. Once Sperm are released, they swim through the Vagina, Cervix, and Uterus, and, finally, up the Fallopian Tubes.   4. If Ovulation occurs anytime from 72 hours before to 48 hours after Ejaculation, sperm may encounter an egg in one of the Fallopian Tubes.   5. Only a Single Sperm is needed to Fertilize an Ovum. Fertilization occurs when a sperm and an egg fuse and form a Zygote.   6. The Ovum is surrounded by a dense protective layer that contains receptor sites to which Sperm bind.   7. This binding causes a Vesicle in the Sperm Head to rupture and release enzymes that break down the protective layer and forms a pathway through which the Sperm Nucleus (Head) can reach the Ovum.   8. Once a Sperm Nucleus (Head) enters the Ovum, the Tail is left outside, the Cell Membrane changes, preventing other sperm from entering the cell.   9. The Fertilized Ovum is properly called a ZYGOTE (2N).  The Zygote will go through Cell Division (Mitosis), and the Ball of Cells (Embryo) will attached itself to the wall of the uterus (IMPLANTATION) and begin to grow into an Embryo - Pregnancy.   8.  Sperm can survive in the female reproductive tract for TWO days or longer, so there are usually three or four days each month when Fertilization is possible.   CLEAVAGE AND IMPLANTATION   1. When an Ovum (Egg) is fertilized, the remarkable process of human development begins.   2. During this process, a single cell no larger than a period at the end of a sentence will undergo a series of Cell Divisions that will result in the formation of a new individual.   3. The first few CLEAVAGES, or MITOTIC Cell Divisions of the Zygote, take place while the Zygote is still inside a Fallopian Tube.   4. Four Days after fertilization, the Embryo consists of a solid ball of about 50 Cells known as the MORULA (MOR-yoo-luh).   5. As the Embryo grows, a fluid-filled cavity forms in the center, transforming it into a BALL of Cells known as a BLASTOCYST.    6. The Morula becomes a Blastocyst by the time it reaches the Uterus.   7. About 6 or 7 Days after Fertilization, the Blastocyst Burrows and Embeds itself to the wall of the Uterus and begins to grow Inward in a process known as IMPLANTATION.  Pregnancy begins at Implantation.   PREGNANCY   1. After Implantation, the Blastocyst slowly takes on the recognizable features of the human infant.   2. This nine month period of development is called GESTATION or PREGNANCY.   3. Pregnancy is divided into Three Equal Periods, or Trimesters.   4. A cluster of Cells gradually forms within the cavity of the Blastocyst.  This cluster sorts itself in to TWO Layers, which then produce a THIRD Layer, by a process of Cell Migration known as GASTRULATION (gas-troo-LAY-shuhn).   5. THE RESULTS OF GASTRULATION IS THE FORMATION OF THREE CELL LAYERS KNOWN AS THE:       A.  ECTODERM - THE OUTERMOST PRIMARY GERM LAYER  IN AN ANIMAL EMBRYO. DEVELOPS INTO THE NERVOUS SYSTEM, EPIDERMIS, AND SWEAT GLANDS.       B.  MESODERM- THE MIDDLE PRIMARY GERM LAYER IN AN ANIMAL EMBRYO.  DEVELOPS INTO THE REPRODUCTIVE SYSTEM, KIDNEYS, MUSCLE, BONES, SKIN, BLOOD, AND BLOOD VESSELS.       C.  ENDODERM- INNERMOST PRIMARY GERM LAYER IN AN ANIMAL EMBRYO. DEVELOPS INTO THE LUNGS, LIVER, THE LININGS OF THE DIGESTIVE ORGANS, AND A SOME ENDOCRINE GLANDS.   6. THESE THREE LAYERS ARE REFEREED TO AS THE PRIMARY GERM LAYERS BECAUSE ALL OF THE ORGANS AND TISSUES OF THE EMBRYO WILL BE FORMED FROM THEM.   7. During Implantation, the Outer Layer of Cells of the Blastocyst produces TWO important Membranes that surround, protect, and nourish the developing Embryo - AMNION AND THE CHORION.    A.  CHORION = PLACENTA - THE CONNECTION BETWEEN MOTHER AND DEVELOPING EMBRYO. THE PLACENTA IS AN ORGAN THROUGH WHICH MATERIALS ARE EXCHANGED BETWEEN THE MOTHER AND THE DEVELOPING EMBRYO.   B.  AMNION = AMNIOTIC SAC - FLUID FILLED STRUCTURE THAT CUSHIONS AND PROTECTS THE DEVELOPING FETUS IN PLACENTAL MAMMALS   8. The Placenta is the Embryo's Organ of Respiration, Nourishment, and Excretion.  The Blood of the Mother and Embryo do NOT MIX; they flow pass each other.   9. The Nine Months of Pregnancy are often divided into THREE MONTH PERIODS KNOWN AS TRIMESTERS.   10.  Most of the Fetus's (After 8 WEEKS) development is completed by the end of the Second Trimester.  During this time, a structure known as the UMBILICAL CORD forms.  The umbilical cord, which contains TWO Arteries and ONE Vein, connects the fetus to the Placenta.   11.  STAGES OF EMBRYONIC AND FETAL DEVELOPMENT:   0-4 WEEKS - Fertilization occurs; embryo travels through Fallopian Tubes and implants in uterine wall; Nervous System, Brain, Digestion System, Ears, and Arms begin to form; Heart Forms and Begins to Beat at 21 Days.   5-8 WEEKS - Nostrils, Eyelids, nose, Fingers, Legs, Feet, Toes, and Bones begin to Form; Females develop Ovaries, Males develop Testes; Head is as large as body; Cardiovascular System is Fully Functional; about 22 mm (less than 1 inch) long.      9-12 WEEKS - Embryo becomes a FETUS, from eight weeks until birth, the developing child is called a Fetus. Penis in Males is distinct; growth of Chin and other Facial Structures give the Fetus a Human Face and Profile; Head is still Dominant, but body is lengthening; about 36 mm (1.5 inches) long.   13-16 WEEKS - Blinking of Eyes and Sucking of Lips occurs; body begins to outgrow Head; Mother can Feel muscular  activity of Fetus; about 140 mm (5.5 inches) long. 17-20 WEEKS - Limbs achieve final proportions; Eyelashes and Eyebrows are present; about 190 mm (6.5 inches) long.   21-30 WEEKS - Substantial increase in weight; may survive if born at this stage; Skin is Wrinkled and Red; about 280 mm (13 inches) long.   30-40 WEEKS - Fingernails and Toenails are present; about 360 mm (14.5 inches) long.       CHILDBIRTH   1. About 9 Months (about 270 Days) after Fertilization, at the end of a Full Term Pregnancy, the Fetus is ready for Birth. By this time, it has usually moved so that its Head is against the Cervix.   2. When is it time?  A Hormone known as OXYTOCIN is released from the Pituitary Gland, that affects a group of large Involuntary Muscles that surrounds the Uterus.   3. WEAK, Irregular Contractions may occur for Several Weeks before birth. (False Labor).   4. As these Muscles are stimulated, they begin a series of Rhythmic Contractions knows as LABOR that Expands the opening of the CERVIX so that it will be large enough (about 10 cm) to allow the baby to pass through it.   5. As contractions continue, they become more Powerful (PAINFUL) and more Frequent, occurring once every minute or two.   6. Little by little, in a process (LABOR) that last from 2 to 20 hours, the baby is FORCED toward the Vagina as labor continues.   7. The Amniotic Sac Breaks (“Breaking Water”), and the fluid it contains rushes out of the Vagina.   8. The baby is finally Forced out of the Uterus and the Vagina, Head First, still attached to its mother by the Umbilical Cord.   9. The Baby will begin to cough or cry in order to rid its lungs of the fluid with which they have been filled.  Breathing starts almost immediately.   10. The Umbilical Cord is clamped and cut, leaving a Scar known as Navel or Belly Button.   11. In the final contractions the Placenta, Amniotic Sac and the Uterine Lining, collectively called AFTER BIRTH are expelled from the mother's body about 10 minutes after the baby is born.   12.   The process of Childbirth is complete.