Several hundred glands in the endocervix produce 20–60 mg of cervical mucus a day, increasing to 600 mg around the time of ovulation. It is viscous because it contains large proteins known as mucins. The viscosity and water content varies during the menstrual cycle; mucus is composed of around 93% water, reaching 98% at midcycle. These changes allow it to function either as a barrier or a transport medium to spermatozoa. It contains electrolytes such as calcium, sodium, and potassium; organic components such as glucose, amino acids, and soluble proteins; trace elements including zinc, copper, iron, manganese, and selenium; free fatty acids; enzymes such as amylase; and prostaglandins. Its consistency is determined by the influence of the hormones estrogen and progesterone. At midcycle around the time of ovulation—a period of high estrogen levels— the mucus is thin and serous to allow sperm to enter the uterus and is more alkaline and hence more hospitable to sperm. It is also higher in electrolytes, which results in the "ferning" pattern that can be observed in drying mucus under low magnification; as the mucus dries, the salts crystallize, resembling the leaves of a fern. The mucus has a stretchy character described as "Spinnbarkeit" most prominent around the time of ovulation.