Gestation is "the act of carrying young in the Uterus, from the time of conception to that of parturition." Conception occurs at the moment of the impregnation of the ovum; parturition is the act of delivery, or childbirth. Pregnancy is "the state of being with child." The terms "period of gestation," and "period of pregnancy," respectively, are employed by medical authorities to designate the time during which the mother carries the young within her own body--from the moment of the impregnation of the ovum until the moment of the final delivery of the child into the outer world. The term of pregnancy in woman continues for over nine calendar months (or ten lunar months)--from about 275 to 280 days, though in exceptional cases it may be terminated in seven calendar months, or on the other hand may continue for ten calendar months. The usual method is to figure 280 days from the FIRST DAY of the LAST MENSTRUATION. A simple method of calculating the probable date of delivery is as follows: COUNT BACK THREE MONTHS, AND THEN ADD SEVEN DAYS, AND YOU WILL HAVE THE DATE OF PROBABLE DELIVERY. Example: A woman's FIRST DAY OF LAST MENSTRUATION is March 28. Counting back three months gives us December 28; and adding seven days to this gives us January 4, as the date of probable delivery. There will always be a possible margin of a few days before or after the ascertained probable date--but the delivery will very closely approximate said date. Ignore the shortage of days of February in this calculation, the same being covered by the general margin allowed. DEVELOPMENT OF THE IMPREGNATED OVUM. In the preceding lesson we terminated our consideration of the impregnated ovum at the point at which, after the process of segmentation, the "primitive trace" had appeared. This primitive trace appears as an opaque streak, or straight line, formed of an aggregation of cells of a distinctive quality. This delicate "trace" or "streak" is the first indication of the form of the coming child. It is the basis, pattern, or mould, in or around which the spinal column is to be formed, and around which the entire young body is to be developed by the wonderful and intricate processes of dividing and reduplication, and the folding and combination of cells. From one end of this "trace" develops the head; from the other end develops the lower end of the spine. At a later stage there appear tiny "buds" in the positions at which the arms and legs should be; these gradually develop, and their ends split into tiny fingers and toes, and finally are transformed into perfect little arms and legs, miniatures of those of the adult human being.