From the genealogico-statistic inquiry made on a group of 252 leukemics hospitalised in Lombardy during the years 1945-1950, and from the comparison made on an adequate material of control; the Authors have come to the conclusion that for the realisation of the leukemia the idiotypic composition has less value than that of the surroundings. They have over more noticed a greater incidence of neoplasiae in the familiars of the acute leukemics and cronic myeloids in regard to the controls.

Observation of the behaviour of a pair of one-egg twins, incompletely concordant with resspect to Paget’s disease, is consistent with the courrent theory (based on 57 families in which the disease has occurred in more than one member of the family, and one other pair of one-egg twins), that the condition is of genetic origin, the gene responsible being an incompletely dominant autosomal gene with variable degrees of penetrance and expressivity.

1. The probability that a mother who has already given birth to twins will repeat the twin birth has been shown to be 3.63 ± 0.24%.

a) The increase in the frequency of binovular twin births of 4,55 ± 0,90% is statistically significant.

b) The increase in the frequency of monovular twin births of 1.43 ± 0,90% is not statistically sure in this data because of the relatively high standard error.

2. Althrough it cannot by proved statistically, there is some indication that, amongst the later births to women who have initially given birth to binovular twins, there is an increase in the frequency of monovular twins, and vice-versa.

3. Hormonal disturbances, such as a slow secretion of folliculin which would prevent ovulation, may be the cause of binovular twinning. Mon ovular twinning is probably due to an inherent tendency of the egg to divide. It is pos sible that if this tendency manifests itself before the reduction division and two separate eggs are formed binovular twinning may result. With this hypothesis is mind, an attempt has been made to discover some correlation between monovular and binovular twinning.

This article discusses the major differences which can be fuond from time to time in homozygotic twins. In some instances there may be differences in manifestation, as in the case of a pair of monozygotic twins, aged 12, with concordant dysplasia of the hipjoint but with different radiological pictures. Differences may also be due to particular incidents during intrauterine life, alterations in growth resulting from hormonal disturbances etc. In some cases the cause of the differences is not known and, as an example, the author cites a monozygotic twin-pair aged 10, one member of which is much retarded in growth while the co-twin suffers from coloboma of the iris.