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THE OCCURRENCE OF `END-TO-END' FUSION BETWEEN ADJACENT CHORIOALLANTOIC SACS IN THE EARLY PIG EMBRYO

In 1922, Corner described three cases of monochorionic twinning in pigs between the 2nd and 4th weeks of gestation. He proposed that these specimens were actually examples of twins arising within a single conceptus. He considered the alternative possibility that each of these three specimens could have arisen by fusion of two originally separate blastocysts unlikely because of the failure to find other and more numerous examples of less deceptively perfect fusions. Further, he stated that such fusion was uncommon in the early stages of development in the pig. However, Hughes (1927, 1929) reported the occurrence of fusion between adjacent chorioallantoic sacs in older pig fetuses (40 to 51 days of pregnancy). She demonstrated that, when there was vascular continuity between the fused sacs of embryos of opposite sex, the female could be