<P> The placenta is used as a means of transferring nutrients from the mother to the fetus . The efficiency at which nutrients are transferred dictates the health and growth of the fetus . FRP, or fetal weight: placental weight ratio, was commonly used to determine placental efficiency . However, increasing FPR does not prove to increase litter size . Instead, a more accurate way of determining fetus growth is through certain characteristics of the placental lining . The placenta is made of a folded trophoblast / endometrial epithelial bilayer . The width and length of the placenta folds are positively related and increase as gestation progresses . </P> <P> The width of the placental folds decreases until day 85 of gestation . From here, the width increases with gestation and is at its largest around day 105 . The rate at which these folds increase is negatively related to fetus size . Thus, greater fold widths will be seen in smaller fetuses . Although increasing placental fold width does increase the interaction between fetus and mother, nutrient exchange is not most efficient in smaller fetal pigs, as would be expected . Many other factors, including depth of placental folds, are also responsible for these interactions . </P> <P> The prenatal development of the fetus includes all the tissue and organ development . Within hours of mating, the sperm and egg undergo fertilization in the oviduct and three days later the egg moves into the uterus . The cells begin to specialize by day six, and attach themselves to the uterus lining by day eleven . From fertilization to day 18, the endoderm, ectoderm and mesoderm have been forming inside the embryo, and are completely formed by day 18, the same day the placenta forms . The endoderm transforms into the lungs, trachea, thyroid gland, and digestive tract of the fetus . The ectoderm has a greater role 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 . </P> <P> 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 . </P>

Where are fetal pigs found growing in their mother