The authors studied the structure of the umbilical cord in a cephalothoracopagus and a holoacardius amorphus. In both cases they met with the absence of one umbilical artery. They discuss on the possible teratogenic influence of this umbilical anomaly, which appears in the acardius as a regular phenomenon, and, along with the velamentous insertion and with the vascular anastomoses bringing about the reverse blood circulation, constitutes the main cause of malformation.

The rate of twinning, both MZ and DZ, as determined by the Weinberg formula, remained fairly static in Scotland up to 1958, when it fell (Fig. 1), due to a fall in the DZ twinning rate. These figures are based on the returns from the Registrar General for Scotland.

As age has a marked influence on the incidence of twinning, it was thought that the difference might be due to a reduced number of older women having babies. However, this does not seem to be the whole answer (Fig. 2). The most marked change would be expected in the women over 35, but this did not occur. It was not possible to look at other factors such as parity, social class and height in the figures for Scotland. The details of all births in the City of Aberdeen are known because of the very high hospital delivery rate, and more detailed studies have been made on the twinning rates for Aberdeen.

These figures, again, showed that from 1959 onwards there was a fall in the twinning rate in Aberdeen and this was due to the fall in the number of DZ twins. The fall in incidence occurred both in Primigravidae and in multigravidae (Tab. I), and more in older than in younger Primigravidae (Tab. II), but there was little difference by age in the fall in multigravidae (Tab. III).

Another factor which is related to the incidence of twinning is the height of the mother. This, in turn, reflects the social class, as tall women are more commonly from the upper, and small women from the lower social classes. The rate of twinning in Aberdeen was highest in the tall women before 1954 (Tab. IV). The rates in all women fell from 1954 onwards, but most dramatically in the tall women. The rates from 1959 onwards show very little difference according to height.

In a girl affected by mental deficiency and slight malformations, a 46,XX, 18r chromosome set was found. Metaphases with irregularly sized or absent rings amounted to 14% in peripheral blood cultures. Tritiated thymidine incorporation pattern in the ring chromosome was similar to that of the normal 18 chromosome.

Agreement prevails that MZ twins have higher concordance rates for cancer than do DZ twins. The exact genetic influence on concordance remains doubtful. Neither the patient groups nor the means by which data are analyzed are comparable. The naturally higher perinatal mortality rate among twins affects statistical analysis of diseases among surviving cotwins. Classification of zygosity among all studies of twins is not entirely reliable.

A review of published reports of cancer among twins notes concordance in a variety of tumors. Genetic factors appear to operate either in the concordance of the cancer or in the site of the specific tumors. A genetic origin of tumors in MZ twins is possible. Tumors, including type, site, and age of onset, affect MZ twins more than both members of a DZ pair.

Computerized permanent twin registers should be established for study of prospective follow-up of large twin populations. Suggestions are made for implementation.

The probability of a twin birth depends strongly and almost equally on the age of the mother and on the number of children she has had. Even when these variables are held constant, the frequency of twinning varies between countries or geographic regions and between years or decades. This variation has been thought perhaps to result from economic and environmental factors, but another explanation can be suggested.

Bulmer (1959) found that twins are frequent among the earliest conceptions after marriage. Eriksson and Fellman (1967) found twins to be frequent among illegitimate births and suggested that the mothers of twins conceive more easily than other women; in other words, that they are more fecundable. We can present some new evidence for this hypothesis, and we believe that high fecundability in the mothers of twins can account for variations in the twinning rate.

Fig. 1 shows the wide range of twinning rates in the US white population, in different parts of the country and for three periods, centering on 1937, 1954 and 1964. The nine geographic divisions are shown in descending order according to the twinning rates in 1937. The two regions at the right, with the lowest rates in 1937, had nearly stable twinning rates over the three time periods. They are the oldest and most urban parts of the country.

A new hypothesis involving two genes has been proposed to explain the nature of hairy pinnae of the ear. The insignificant values of the goodness of fit χ2 seem to confirm the proposed hypothesis.

A twin population model is proposed, and two different definitions of the rate of concordance are investigated within the context of this model. It is found that the statistic formed from the ratio of the number of affected twins that come from concordant pairs to all affected twins has certain advantages over the traditional definition of the rate of concordance. This twin population model is extended to nontruncated sample of twins. The parameters of this situation are estimated. Finally, it is shown how these parameters relate to the two usual estimates of the proportion of DZ. twins in a twin population, and the efficiency of these methods is computed.

An unusual ear type in man involving a folding of the helix is described in a pedigree with three generations. The trait seems to be due to an autosomal dominant gene with variable expressivity.

In Bulgaria, in un periodo di 88 anni, sono nati circa 12 500 000 bambini, di cui circa 300 000 gemelli, 4000 trigemini e 100 quadrigemini. Si è dunque avuta una frequenza di 1/80 per i parti gemellari, 1/6 000 per quelli trigemini, e 1/500 000 per quelli quadrigemini. In 69 anni (1894-1962) si sono verificati 23 parti quadrigemini, cioè, uno ogni tre anni.

Fino al 1912, la frequenza dei parti gemellari è stata dell'1.5%; 50 anni più tardi (1962), tale frequenza diminuisce della metà. Dal 1904 al 1912, su una popolazione di 60 00 000 di abitanti, sono nati 168 000-186 000 bambini all'anno; dal 1954 al 1962, su una popolazione di 8 000 000 di abitanti, ne sono nati 135 000-150 000.

La brusca diminuzione della natalità gemellare nel paese è più indicativa se si fa il confronto su 10 000 nati. Per i maschi, la frequenza cade da 221/10 000 a 165/10 000; per le femmine, da 234/10 000 a 170/10 000.

Twinning is regarded as a very important phenomenon among the Yorubas in Western Nigeria. In this population, special ceremonies are performed to mark twin deliveries, and twin babies and their parents are given special names. Twins are therefore more readily identified, and data relating to twinning in families are likely to be fairly accurate.

Furthermore, the practice of polygamy in Western Nigeria provides a good opportunity for studying both maternal and paternal factors in the inheritance of twinning. The high incidence of twinning in the population (45‰: Nylander, 1969) is also an additional advantage, since it allows a large number of twin maternities to be studied in a short space of time.

Among 18 737 deliveries in the three major hospitals in Ibadan, Western Nigeria, between October 1967 and June 1969, there were 977 twin maternities. Every mother was interrogated immediately after delivery to find out whether she or her husband was a twin or had twins previously (including those delivered by the husband's other wives), and whether she or her husband had twin siblings. Obstetric data, including maternal age and parity, were collected for each patient, and zygosity of the newborn twins was determined by sex, placentation, blood groups, placental enzymes, haemoglobin types, and G6PD electro-phoretic studies, as will be described in a subsequent paper.

Twinning rates in Nigeria are much higher than in Europe and USA, and appear to be highest in its western and eastern parts. The twinning incidence in a representative population in a town in Western Nigeria, Igbo-Ora, is approximately four times that in Europe and USA.

Zygosity determination has shown that only 8.6% of newborn twin pairs in Ibadan are MZ, compared to approximately 30% in Caucasian populations. Studies in Igbo-Ora have also given similar results, confirming the view that high twinning incidence in Nigeria is due to a very high DZ twinning rate, the MZ rate being the same as in Europe and USA.

The proportion of twins with monochorionic placentae in Ibadan is about 5%, against approximately 20% in UK and USA. The twinning rates in the two populations are 45‰ and 11‰ respectively. This inverse relation (between the twinning rate and the proportion of monochorionic placentae) may be used in calculating the twinning incidence in a population in which the proportion of twins with monochorionic placentation is known.

The high rate of twinning in rural areas is suggested to be due to higher coefficient of inbreeding, better physical conditions of mothers on the countryside and higher mean birth order. Family data indicate the frequent role of maternal genotype in DZ twinning.

Nelle ricerche sulla gemellazione la madre dei gemelli è la candidata, ma il carattere ereditario si manifesta nella generazione successiva.

Partendo da considerazioni probabilmente diverse, molto studio è stato dedicato direttamente alla candidata, ai suoi caratteri fisici e fisiologici. La generazione successiva è stata invece alquanto trascurata, sicché, quando abbiamo voluto considerare l'andamento del peso alla nascita dei fratelli di gemelli, non ci è stato possibile rin-tracciare precedenti bibliografici.

Il peso alia nascita è sotto l'influenza di innumerevoli fattori ed ovviamente non è un carattere ereditario. Ma, se la gemellazione è provocata da un sistema ormonico controllato dal genotipo, è probabile che questo sistema agisca anche sul peso alla nascita dei fratelli di gemelli.

Frequency and inheritance of twinning have been studied from urban and rural parts of Madhya Pradesh. Family data conform to previous findings by other authors, for other countries, that DZ twinning is influenced by the mother's genotype.

Of special interest has been a rural family in which a female, herself a cotwin of DZ pair, married twice, each time giving birth to DZ twins.

The high rate of twinning in rural areas is suggested to be due to higher coefficient of inbreeding, better physical condition of mothers on the countryside, higher mean birth order, and higher mean age of mothers. Lack of family planning may have also played a role.

Skin reflectances were obtained at 685 mμ wave length from the medial upper arm and the forehead regions among 105 Tibetan males and 105 Tibetan females, in order to study sex differences in human skin pigmentation. The forehead pigmentation of males was always found to be darker than that of females, and the difference to be significant during middle and late adolescence The medial upper arm pigmentation of girls was found to be darker than that of boys, during early adolescence; but during middle adolescence the two sexes exhibited just similar pigmentation and during late adolescence females emerged to be significantly lighter than males. The present study, thus, suggests that one of the factors causing sex differences in human skin pigmentation may be the physiological changes occurring during adolescence, which may be genetically determined.

Nos explications ont pour but de faciliter la compréhension de la concordance et surtout la discordance des jumeaux MZ et DZ moyennant les analyses chromosomiques. Dans les dernières années tout un nombre d'études concernant les jumeaux ont été publiées où chaque jumeau accusait le même phénotype et caryotype. Il s'agit de jumeaux MZ avec les syndromes de Down (Mikkelsen et Melchior, 1962), de Klinefelter (Bertrand et al, 1966; Holub et al, 1958; Nowakowski et al, 1963; Vague et al, 1968) et de Turner (Decourt et al. 1964; Frasier et al, 1961; Nance et Uchida, 1964; Turner et Zanartu, 1962); aneuploïdies doubles (Hustinx et al, 1961; Turpin et al, 1964); mosaïques gonosomiques (Mikkelsen et al, 1963) et caryotype XYY (Rainer et al, 1969).

Le problème de la discordance des jumeaux MZ nous préoccupe davantage que celui de la concordance, car en effet la discordance a été l'objet de maintes études. Le résultat n'est pas satisfaisant pour tous les jumeaux étudiés (Nielsen, 1967; v. Verschuer, 1927). À ce sujet, je tiens à rappeler la discussion qui a eu lieu dans l'Institut Mendel, au cours du Premier Symposium International de Génétique Médicale du 6 au 7 septembre 1953, entre Kallmann et v. Verschuer. Parmi les premiers savants il faudrait citer Meirowsky (1926) qui expliquait les causes de la discordance par un partage inégal du plasma lors du clivage du disque embryonnaire.