developing embryo

• While blastocyst is implanting in lining of uterus, inner cell mass undergoes changes as cells change to multipotent 

  • Results in the formation three layer - primary germ layers 
• Ectoderm : 

  Outermost germ layer

  Form outer layers of body

  • Skin, hair, mammary glands, nervous system 
• Mesoderm : 

  Middle germ layer 

  Skeleton, muscles, connective tissue, heart, blood, stomach, intestine
• Endoderm : 

  Innermost germ layer

  Lining of digestive system, lungs, thyroid 
• Embryonic membrane 

  Membranes that lie outside embryo and serve to protect and nourish it as it develops

  • Four layers early in embryonic period 
• Amnion : 

  First membrane to develop

  By eighth day, it surrounds the embryo enclosing a cavity into which is secrete amniotic fluid 

  • Protect embryo against physical injury - acts as shock absorber 
  • Helps maintain a constant temperature 
  • Allows developing embryo / foetus to move freely 

  Expands as growth takes place 

  Usually ruptures just before childbirth, releasing amniotic fluid - water break 
• Chorion : 

  Formed from out cells of blastocyst and mesodermal cells 

  Surrounds embryo and other three embryonic membrane 

  Inner layer of chorion fuses with amnion as it enlarges

  Eventually becomes main part of foetal portion of placenta
• Yolk sac and Allantois : 

  Not as important in humans 

  Form outer structure of umbilical cord
• Placenta 

  Organ that forms from both foetal and maternal tissues during first three months of gestation 

  • Foetal portion developing from part of chorion
• Endocrine 

  Secretes number of hormones necessary to maintain pregnancy
• Excretory 

  Transports nitrogenous waste (urea, uric acid, ammonia, creatinine) from foetal blood to mother’s blood for excretion by mother’s kidneys 
• Immune 

  Transports antibodies from mother into foetal blood supply to foetus has immunity to some infectious diseases 
• Nutritional 

  Transport nutrients (glucose, amino acids, fatty acids, minerals) from mother’s blood to foetal blood

  Stores some essential nutrients early in pregnancy and releases them later when demand is greater
• Respiratory 

  Transports  oxygen from mother to foetus and carbon dioxide from foetus to mother 
• Foetal part begins developing as blastocyst is implanted in endometrium 

  Small, branching, finger-like projections (chorionic villi) develop from chorion

  • Contain numerous blood vessels 
  • Grow into endometrium 

  Villi become surrounded by pools of mother's blood which collect in spaces within endometrium 

  • Villi are bathed in mother’s blood 
• Placenta is attached to foetus by umbilical cord 

  • Two umbilical arteries - blood to capillaries of chorionic villi 
  • One umbilical vein - blood from placenta to foetus 

  Blood from mother enters through uterine arteries, flows through blood spaces (exchange), leaves through uterine veins