During the period of the embryo, the becomes the nervous system, the becomes the circulatory system, and the becomes the digestive system

Weeks 3-4: three layers form: the ectoderm, the mesoderm, and the endoderm. Neural tube develops and produces neurons. Heart begins beating. Ribs, muscles, and digestive tract form. weeks 5-8: arms and legs develop, then fingers and toes; placenta and umbilical cord function; digestive system develops; liver produces blood cells. 

the abdomen (less formally called the belly, stomach, tummy or midriff) constitutes the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. the region occupied by the abdomen is termed the abdominal cavity. the stomach secretes acid and enzymes that digest food. ridges of muscle tissue called rugae line the stomach. the stomach muscles contract periodically, churning food to enhance digestion. the pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine.

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