LITERATURE on prenatal growth in mammals is reviewed. A comprehensive series of measurements was made on the genital tracts of Merino ewes, 11 non-pregnant and 38 at various stages of pregnancy, and also on the foetuses. Data on the endocrine and mammary glands are also presented. During pregnancy the entire genital tract (except the Fallopian tubes) enlarges by active growth, which occurs earlier in the body and horns of the uterus than in the vagina and cervix. The placenta reaches its maximum about midterm and then decreases in weight. The fetal membranes increase steadily in weight throughout pregnancy. Amniotic fluid increases rapidly in volume until the 3rd mth., then decreases, but this decrease is balanced during the 4th mth. by increase in volume of allantoic fluid, which continues to full term to give an accumulation of total fluid during the 5th mth. The mammary gland begins to increase in weight in the 4th mth., but the major growth occurs in the last mth. Large individual variations were found in the growth of the maternal endocrine glands. Foetal length and weight growth can be represented by: LogeDimension = a+bLogeAge+c(LogeAge)2. There is no abrupt break in % growth rate curves of length and weight; length of vertebral column is the most satisfactory length measurement, while because of its high coefficient of variation, the weight/length ratio has little value as an age index. The theory is advanced that the proportions of the growing fetus change as a result of differential rates of retardation of regional growth, retardation in any region being proportional to the lapse of time since growth began there. Correlation coefficients between all pairs of measurements are given; in all cases total correlations are high and significant but are due almost entirely to the common growth trend. A simple nomogram is presented by which age may be read from one or more dimensions. The fetus tends to be carried in the horn corresponding to the ovary with the corpus luteum; there is no significant difference between numbers carried in each horn. An approximate correction for the weight of the fetal system in pregnant ewes may be made by the formula: Nett weight of ewe = Gross weight-Antilog (-3.0337+2.3891 Loge Gestational Age)
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