Developing embryo. Artwork showing the development of an embryo from the third week of development until it reaches the foetal stage (nine weeks). At the third week a process known as gastrulation occurs, forming three types of tissue known as germ layers; the endoderm, mesoderm and ectoderm. From 3-8 weeks these germ layers develop into tissues and organs. The nervous and vascular systems, gastrointestinal tract, skin, kidneys and respiratory system are formed. The embryo assumes a rounded body, its limbs and digits form and it develops a human-like face. The ninth week is the beginning of the foetal period. By now the foetus is 5-8 centimetres long and weighs 10-45 grams.