Growth and size depends on environmental conditions as well as genetic endowment. The first year is characterized by rapid physical growth. A normal baby doubles its birth weight in six months and triples in a year. During that time, there is great expansion of the head and chest, thus permitting development of the brain, heart, and lungs, the organs most vital to survival. The bones, which are relatively soft at birth, begin to harden, and the fontanelles, the soft parts of the newborn skull, begin to calcify, the small one at the back of the head at about 3 months, the larger one in front at varying ages up to 18 months. Brain weight also increases rapidly during infancy: by the end of the second year, the brain has already reached 75% of its adult weight. The conditional growth symptoms also includes short stature, growth retardation, delayed growth, lack of growth, severe dwarfing, slow weight gain and slow height gain.