Some effects of antigenic dissimilarity of mother and conceptus on placental and foetal growth, and on gestation period were investigated. Mothers were immunized against or rendered tolerant to the foreign antigens of the hybrid foetus. Placentae and foetuses were weighed from the 12th day of pregnancy up to immediately before term. Some of the litters were allowed to come to term and recordings were made of the birth weights and the length of the gestation period.
The results indicate that immunological factors have a significant effect on the size of the placenta and on the growth rate of the foetus. Probably because of this the period of gestation was altered. It was concluded that a unique balance is struck between the transfer area of the placenta and the mass of foetal tissue, to the detriment of the foetus if the placenta is unduly small, but to its advantage if a larger placenta is formed.
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