The configuration of the posterior bifurcation of the posterior communicating artery is commonly described as the 'adult configuration' if the diameter of the precommunicating part of the posterior cerebral artery (P1) is larger than the diameter of the posterior communicating artery itself (PCA). In these cases the blood supply to the occipital lobes is mainly from the vertebro-basilar system. Only in a minority of cases is the fetal or embryonic configuration found. Here the diameter of the PCA is larger than the diameter of the P1 and the blood supply to the occipital lobes is mainly from the internal carotid artery via the posterior communicating artery. In order to track the origin of these different configurations, 53 complete circles of Willis (106 sides) in brains of fetuses and infants aged from 12 weeks to 60 weeks after conception were examined with the aid of an operating microscope, and measurements were made of the diameters of the PCA, P1 and P2. It is concluded that the variations of this part of the circle of Willis are the result of developmental modifications. This is confirmed by a statistical analysis that shows a relation between the stage of development of the brain and the occurrence of the different configurations. The frequencies of the adult and fetal configurations gradually increase at the expense of the transitional configurations. This configuration is found early in development and is characterised by the equality of the diameters of the PCA and P1. These findings contradict the views, found in the literature, that the variations either exist early in development as a consequence of genetic factors or arise after birth as a consequence of mechanical genetic factors.