in the development of the fetus. It forms into the skin, nervous system, enamel of the teeth, lining of the intestine, mammary and sweat glands, hoofs, and hair. The mesoderm forms the major organ components that help keep the fetus alive. It forms the muscles and connective tissues of the body, blood vessels and cells, the skeleton, kidneys, adrenal glands, heart, and the reproductive organs. By day 20, most of the major organs are visible, and the last half of gestation focuses greatly on increasing the size of the fetuses. Development of lymphoid and haematopoiesis tissues The development of the lymphatic system and the formation of blood circulation occur at different stages of fetal pig development. The first lymphatic organ to become present is the thymus. Lymphocyte builds up in the spleen on the 70th day. By day 77, the thymus is already completely developed and is distinguishable from other organs. Also, follicles are present on the tongue and intestines on day 77. On the 84th day, Periarteriolar lymphoid sheaths appear in the fetal pig. By this time, the liver and bone marrow are active and functional. Environmental effects on swine reproductive performance Studies have shown that litter size, the amount of floor space during the growing period, and the number of pigs the gilt, or female pig, is placed with while growing affect the reproduction rates of the gilts. Data from a study in 1976 by Nelson and Robinson showed that gilts from a small litter size ovulated more than the gilts from the larger litters. The study suggests stress plays a role in impacting the reproduction. The amount of floor space has been shown to impact the time it takes gilts to reach puberty. An adequate amount of floor space allowed the higher percentage of gilts to reach puberty sooner than those gilts who had less floor space. The gilts placed in smaller groups bore one more pig per litter than gilts in larger groups. Still, the environment in which the fetal gilt develops is significant to the reproductive and physiological development. Preservation Fetal pigs are often preserved in formaldehyde, a carcinogenic substance. A 1980 study found that exposure to formaldehyde could possibly cause nasal cancer in rats, leading to research on whether this was possible in humans or not. In 1995 it was concluded by the International Agency for Research on Cancer (IARC) that formaldehyde is a