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• Organs form from the germ layers through the process of differentiation. During differentiation, the

  embryonic stem cells express specific sets of genes which will determine their ultimate cell type.
• The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube.
• In further development, the neural tube will give rise to the brain and the spinal cord.
• In vertebrates, one of the primary steps during organogenesis is the formation of the neural system.
• During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells
• The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal cord.
• A spatial pattern of gene expression reorganizes the mesoderm into groups of cells called somites with spaces between them.
• The somites, will further develop into the ribs, lungs, and segmental (spine) muscles.
• The mesoderm also forms a structure called the notochord, which is rod-shaped and forms the central axis of the animal body.
• In this five-week old human embryo, somites are segments along the length of the body.
• Even as the germ layers form, the ball of cells still retains its spherical shape. However, animal bodies have lateral-medial (left-right), dorsal-ventral (back-belly), and anterior-posterior (head-feet) axes,
• In one of the most seminal experiments ever to be carried out in developmental biology, Spemann and Mangold took dorsal cells from one embryo and transplanted them into the belly region of another embryo. They found that the transplanted embryo now had two notochords: one at the dorsal site from the original cells and another at the transplanted site
• Pregnancy begins with the fertilization of an egg and continues through to the birth of the individual.
• The length of time of gestation varies among animals, but is very similar among the great apes: human gestation is 266 days, while chimpanzee gestation is 237 days, a gorilla’s is 257 days, and orangutan gestation is 260 days long.
• The fox has a 57-day gestation. Dogs and cats have similar gestations averaging 60 days. The longest gestation for a land mammal is an African elephant at 640 days. The longest gestations among marine mammals are the beluga and sperm whales at 460 days.
• Twenty-four hours before fertilization, the egg has finished meiosis and becomes a mature oocyte.
• When fertilized (at conception) the egg becomes known as a zygote
• The zygote travels through the oviduct to the uterus .The developing embryo must implant into the wall of the uterus within seven days, or it will deteriorate and die.
• Another layer of the blastocyst, the chorion, begins releasing a hormone called human beta chorionic gonadotropin (β-HCG) which makes its way to the corpus luteum and keeps that structure active
• Pregnancy tests determine the level of β-HCG in urine or serum. If the hormone is present, the test is positive.
• In humans, fertilization occurs soon after the oocyte leaves the ovary. Implantation occurs eight or nine days later.
• The gestation period is divided into three equal periods or trimesters.
• During the first two to four weeks of the first trimester, nutrition and waste are handled by the endometrial lining through diffusion.
• As the trimester progresses, the outer layer of the embryo begins to merge with the endometrium, and the placenta forms. This organ takes over the nutrient and waste requirements of the embryo and fetus, with the mother’s blood passing nutrients to the placenta and removing waste from it.
• Chemicals from the fetus, such as bilirubin, are processed by the mother’s liver for elimination. Some of the mother’s immunoglobulins will pass through the placenta, providing passive immunity against some potential infections.
• Internal organs and body structures begin to develop during the first trimester.
• Some of the mother’s immunoglobulins will pass through the placenta, providing passive immunity against some potential infections.
• Internal organs and body structures begin to develop during the first trimester. By five weeks, limb buds,

  eyes, the heart, and liver have been basically formed. By eight weeks, the term fetus applies, and the body

  is essentially formed.
• The individual is about five centimeters (two inches) in length and many of the organs, such as the lungs and liver, are not yet functioning. Exposure to any toxins is especially dangerous during the first trimester, as all of the body’s organs and structures are going through initial development. Anything that affects that development can have a severe effect on the fetus’ survival.
• During the second trimester, the fetus grows to about 30 cm (12 inches), It becomes active and the mother usually feels the first movements. All organs and structures continue to develop.
• During the third trimester, the fetus grows to 3 to 4 kg (6 ½–8 ½ lbs.) and about 50 cm (19–20 inches) long, . This is the period of the most rapid growth during the pregnancy. Organ development continues to birth (and some systems, such as the nervous system and liver, continue to develop after birth).
• Third trimester -The mother will be at her most uncomfortable during this trimester.
• There is rapid fetal growth during the third trimester
• Labor is the physical efforts of expulsion of the fetus and the placenta from the uterus during birth (parturition).
• Toward the end of the third trimester, estrogen causes receptors on the uterine wall to develop and bind the hormone oxytocin.
• The oxytocin causes the smooth muscle in the uterine wall to contract.
• There are three stages to labor. During stage one, the cervix thins and dilates. This is necessary for the baby and placenta to be expelled during birth. The cervix will eventually dilate to about 10 cm. During stage two, the baby is expelled from the uterus. The uterus contracts and the mother pushes as she compresses her abdominal muscles to aid the delivery. The last stage is the passage of the placenta after the baby has been born and the organ has completely disengaged from the uterine wall.
• If labor should stop before stage two is reached, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor
• An alternative to labor and delivery is the surgical delivery of the baby through a procedure called a Caesarian section.
• Prolactin causes the mammary glands to produce milk.