Twins

TWINS, the name for two or several offspring born simultaneously by a female, who usually bears one offspring. In humans, the percentage of twin births constitutes 1.1% of all normal births. This percentage is made up mainly of twins, since triplets are born very rarely (1 in 8,000 normal births), and cases of a woman giving birth to 4 or 5 twins already represent the rarest exceptions. According to statistical data by Marsh, this percentage varies for individual nationalities, reaching 1.5% for some of them; it is possible that racial differences play some role here. Twins are divided into a group of dizygotic and a group of monozygotic twins. The mechanism of origin and the factors determining the appearance of the two groups are completely different, and therefore they must be considered separately for both groups. Dizygotic twins are called dissimilar twins, whose similarity is no greater than ordinary sibling resemblance. They originate from two or more simultaneously ripening and simultaneously fertilized eggs, which can be anatomically proved by the presence in such cases of two or more corpora lutea in the ovary, whereas in a normal single pregnancy there is always only one corpus luteum in the ovary. Dizygotic twins can be of different sexes, which occurs in 50% of cases. Observations by Lillie on bisexual twins in cows are interesting in this regard. When these animals give birth to bisexual twins, one of them, usually a female, is abnormal and carries traits of hermaphroditism. These abnormal bisexual twins, known under the name of "freemartins," occur due to unfavorable conditions of intrauterine development, complicated by the interaction of male and female hormones, thanks to the presence of vascular plexuses between the two twins. In humans, such plexuses have not been found, although it is possible that in some cases here there are conditions for such hormonal influences. But, with the exception of such anomalies, dizygotic twins differ from normal births and from ordinary brothers and sisters only in that they go through the embryonic cycle of their life together. In embryonic life they are surrounded by various membranes, internal and external. Placentas are either completely separate or more or less fused with each other, which depends on the site of implantation of the eggs in the tissue of the uterine membrane. In those cases when these points are close to each other, fusion of the placenta often occurs. Monozygotic, or identical, twins are called such twins which occur as a result of the division of one fertilized egg into two or more parts, from which each develops into a fully independent organism. Such twins are genetically completely identical and, being always of the same sex, are so similar to each other externally that even close relatives mix them up. In embryonic life they have a common placenta and common chorion; amnions, however, are usually separate. Therefore, histological analysis of the embryonic membranes at the moment of birth of twins is the most accurate criterion for separating them into monozygotic and dizygotic twins. The mechanism of origin of monozygotic twins is still not sufficiently clear. Until recently, many authors (Schwalbe, Broman, Hacker, Herbst, etc.) tended to consider monozygotic twins as a degenerative symptom, as a result of pathological, mainly toxic, influences in the first time of intrauterine life. In this regard, many consider syphilis one of the main causes (Fournier, Mercklen). Hence the representation of monozygotic twins as biologically inferior individuals (Yudin, Grothier, Kalms). But in recent times, the problem of the origin of monozygotic twins receives a completely different illumination on the basis of works by Peterson and Newman on the origin and development of monozygotic twins in the American nine-banded armadillo; this small vertebrate is remarkable in that it always ripens and fertilizes one egg, which then divides into parts, usually four, giving rise to four monozygotic twins. According to Peterson and Newman, there is always a case of delayed placentation of the fertilized egg, which after fertilization does not immediately attach to the mucous membrane of the uterus and remains for some time in a period of rest; during this period the division of the egg into several parts occurs, from which the twins are formed. According to observations by Stritter and Stockard, the development of monozygotic twins in humans is also the result of mechanical delay of placentation of a normally fertilized egg. The idea of the pathological nature of monozygotic twins themselves is not confirmed by newer researchers. Thus, according to data by Siemens, Verschuer, Bunak, and others, monozygotic twins are fully normally developed subjects, without deviations from the norm in physical or psychological terms. The tendency to the appearance of twins is hereditary. Since the mechanism of origin of twins is different for monozygotic and dizygotic twins, then hereditary predispositions to the appearance of the one or the other must be completely different. Weinberg considers that the inheritance of the tendency to produce twins exists exclusively in relation to dizygotic twins, and the hereditary transmission of this phenomenon occurs only from the mother's side; the father plays no role here. The mechanism of this hereditary transmission is obviously determined by the peculiarities of the female ovary in the sense of simultaneous ripening and release, instead of one, two or several eggs. Other authors (Davenport, Bunak) consider that the hereditary tendency to produce twins exists equally for monozygotic twins. Insofar as delayed placentation is considered as the cause of the occurrence of monozygotic twins, insofar as the peculiarities of the structure of the mucous membrane of the uterus, for example, its different sensitivity to irritation by the fertilized egg, or the peculiarities of the latter, can be factors producing twins and transmitted to offspring. Hypotheses have been proposed by Davenport and other authors according to which certain hereditary properties of the father's sperm may contribute to the birth of twins. The study of twins represents very great interest. Especially interesting are monozygotic twins, such as people with absolutely identical hereditary predispositions. Therefore, the study of monozygotic twins is indispensable in those cases when it is necessary to decide whether a given disputed trait is determined by heredity or is caused by some unknown external factors to us. Hence the enormous importance for medicine of the study of the pathology of monozygotic twins for clarifying the endogenous or exogenous origin of those or other pathological variations (Siemens).