TWINS AND TWINNING. That human twins are of two kinds is now well established. Most mammals regularly or occa sionally bear several young at a litter, and these are taken to arise from as many eggs matured together and ready for fertiliza tion at one heat. Human twins of this kind are derived from two distinct eggs. In the nine-banded armadillo (Dasypus novem cinctatus texanus) and related species, on the other hand, a to tally different cause of multiple births has been found. A single embryo is found at a certain early stage to divide and subdivide into four parts, which develop severally into four distinct off spring. In addition to producing two-egg twins the human race produces also a considerable proportion of twins derived from a single egg in a manner probably analogous to the armadillo. Man is the only mammal known to produce both kinds.
In most civilized countries account is taken of multiple births in official birth registration. These official records show approxi mately one in Ioo twin births, one in io,000 triplets and one in 1,000,000 quadruplets. The exact proportions vary from place to place and from period to period, but owing to the higher pro portion of premature and still-births, and to variability in the rates of survival and in the completeness of registration, it is not possi ble safely to make international comparisons of frequency, or to discover whether the frequency is increasing or decreasing. The mortality at and soon after birth increases rapidly with increas ing number born. A recent British enquiry shows that of triplet children, about half the girls and three-eighths of the boys sur vive ; the chance of life of quadruplets is much lower. For the notable case of Canadian quintuplets see DIONNE QUINTUPLETS.
Very exceptionally twins are joined at birth; such junctions are said to be effected always between like parts, with respect to which the twins are placed symmetrically. Superficial unions are easily severed at birth. More deeply united pairs seldom survive, though
a few such have become celebrated for their peculiarity. Joined twins usually exhibit an abnormal disposition of the blood-vessels, known as situs inversus, in which the normal differences between the right and left sides of the body are in one twin at least par tially reversed. This condition is not found in separate one-egg twins. Joined twins are always of the same sex, and evidence points to their being always one-egg twins in which fission has re mained incomplete. That separate one-egg twins show symmetry reversal of palm and finger pattern has not been established. At birth an expert can distinguish one-egg from two-egg twins by an examination of the foetal membranes in the afterbirth. In the first type the chorion is believed always to be single, in the second double. Although earlier accounts of the proportion of the two conditions showed serious discrepancies, the data from more modern examinations agree well with the proportion judged to be single-egg twins from the sex distribution.