A section through a eucaryotic cell, with its prominent nucleus, in which the long DNA molecules (carrier of genetic instructions) lie tangled up. Other prominent cell components or organelles are floating in the endoplasmic fluid between the nucleus and the outer cell wall. These organelles include the mitochondria (responsible for energy budget), lysosomes (digestion), the reticulum and the Golgi apparatus. The human body is made of eucaryotic cells, about 100,000,000,000 of them. A comparison of vertebrate embryos at thesame stage of development. (Courtesy: G. Romanes)