16 March 2014

#116 The development of the fetus

Credit: thomasreynoldsblog
The blastula develops into an embryo and some of the cells form a placenta, linking the embryo with the uterus lining. Organs such as the heart develops and, after 8 weeks, the embryo is called a fetus.

Growth of the fetus requires a good supply of nutrients and O2. This is achieved through the link between the placenta and the mother’s blood supply in the uterus lining. The placenta is soft and dark res, and has finger-like projections called villi. The villi fit closely into the uterus wall. 

1. Umbilical cord 
  • joins fetus to placenta; contains:
  • 2 arteries: blood from fetus ---> placenta
  • 1 vein: returns blood ---> fetus
2. Placenta 

Brings blood supply of fetus close to mother’s

  • Blood from the fetus passes through the umbilical cord in the umbilical artery to the placenta. 
  • Here it comes close to the mother's blood. 
  • transport O2 + nutrients (amino acids, glucose…) from mother ---> fetus
  • transport CO2 + wastes (urea… ) from fetus --->mother (through umbilical vein).

Prevents mixing 

This is really important because the fetus and mother may have different blood groups - any mixing could result in blood clotting, which could be fatal to both mother and fetus. 

3. An amnion protects the fetus

The fetus is surrounded by a strong membrane, called amnion. Inside the amnion is a liquid called amniotic fluid.

The fetus in the amniotic sac. Credit: preventdisease.com

Amniotic sacmembrane from embryo cells: encloses fetus, prevents entry of bacteria

Amniotic fluid:  supports, protects fetus from mechanical damageabsorbs urine released by fetus.

Try this 


a) i) A, umbilical cord ; B, vagina
     ii) Tree functions from:
      - transfers O2 from mother to fetus
      - transfers nutrients (of named nutrients) from mother to fetus
      - transfers CO2 from fetus to mother
      - transfers wastes (or named wastes) from fetus to mother  
      - allows the transfer of antibodies from mother to fetus
      - prevents mixing of the blood of mother and fetus.

     iii) Helps prevent bacteria passing from mother to fetus, the blood group of mother and fetus may be different. 


  1. There is a mistake in the answer 1ii) about the placenta. The placenta transfers waste from fetus to the mother not the other way round. Indeed. Excellent site. Thank you very much.

  2. Of course you are right. The silly error has been corrected. Thanks.