human development

• Pregnancy or Gestation

  • begins at conception, the fertilization of the egg by a sperm and continues until the birth of baby.
  • lasts about 266 days (38 weeks)
• Cleavage

  • 24 hours after fertilization
  • produces 100 cells
  • embryo becomes blastocyst - hollow ball
• Blastocyst

  • mammalian equivalent of sea urchin blastula
  • fluid-filled cavity that will form baby
• Embryo reaches uterus by 6th or 7th day after fertilization
• Trophoblast

  • outer layer of cells
  • secrete enzymes that enable the blastocyst to implant in endometrium (uterine lining)
• Trophoblast cells eventually form part of the placenta
• Placenta 


  organ that provides nourishment and oxygen to the embryo and helps dispose of its metabolic wastes.

  also produces blood cells and first germ cells

  allows protective antibodies but can also be a medium for viruses like HIV
• Gastrulation

  • 9 days after conception
  • extraembryonic membranes develop
• Extraembryonic membranes 


  develop as attachments to the embryo and help support it
• Amnion, yolk sac, chorion


  blue cells, yellow cells, trophoblast
• Ectoderm, mesoderm, endoderm 


  Embryonic layers
• Ectoderm (blue)


  outer part of embryo's skin; continuous with amnion
• Endoderm (yellow)


  embryo's digestive tract; continuous with yolk sac
• Mesoderm (pink) 


  bulk of most other organs
• Amniotic cavity


  filled with fluid which protects embryo
• First trimester

  • month old-human embryo: 7mm (0.28) in; somite stage frog embryo
  • fetus - 9 weeks after fertilization
  • the clear sac around the fetus is the amnion
  • it is about 5.5 cm (2.2in) long and has all of its organs and major body parts, including a disproportionately large head.
  • the somites have developed into segmental muscles and the bones of the back and ribs.
  • the limb buds have become tiny arms and legs with fingers and toes.
  • sex of fetus is evident by end of 1st trimester
• The main developmental changes during the second and third trimesters involve an increase in size and general refinement of the human features
• Allontois


  fourth extraembryonic membrane, extension of yolk sac
• embryo develops from the three inner layers
• amnion breaks before childbirth and amniotic fluid leaves mother's body through vagina
• yolk sac in humans contain no yolk, yolk sac in birds or reptiles do
• allantois


  forms part of the umblical cord and embryo's urinary bladder
• chorion

  • outermost membrane
  • becomes part of placenta
  • secretes HCG
• human chorionic gonadotrophin 


  hcg, maintains corpus luteum during first 3 months
• corpus luteum 


  secretes estrogen and progesterone which stop menstruation
• chorionic 


  outgrowths larger and contain mesoderm, embryonic blood vessels, and maternal blood
• chorionic villi 


  absorbs nutrients and oxygen from blood and passes to embryo via embryonic blood vessels

  • blue carries wastes away
• HCG is only present when pregnant