The visual pathways of eutherian mammals and marsupials develop according to a common timetable

Data from cats, ferrets, hamsters, macaques, mice, rabbits and rats concerning the time of occurrence of 26 developmental events involved in the establishment of the retinofugal, geniculocortical, corticogeniculate and corticocollicular pathways were analysed. For each species the timing of developmental events was expressed as a proportion of the period between conception and eye opening ('caecal period'). For 6 of these events, the values for all species fell within a range of 1-5% of the caecal period, the values for 11 other events fell within a range of 6-10% of the caecal period, 4 events had ranges of 11-15% of the caecal period, and only 5 events were spread over more than 15% of the caecal period. The 26 events had an overall mean variation of about 11% of the caecal period, suggesting that the visual pathways of eutherian mammals develop according to a common 'timetable' that is related to the duration of the caecal period. One of the roles of this common timetable may be to assist the establishment of orderly interconnections between the visual centres. Relatively few data are available concerning the timing of developmental events in marsupial visual pathways. However, it is apparent that, during the first half of the caecal period, most events occur much earlier in marsupials than in eutherians, whereas during the second half of the caecal period most events occur at the same stages of the caecal period in both marsupials and eutherians. The accelerated rate of visual development during the first half of the caecal period in marsupials may be related to the precocious development that they undergo prior to their very early birth.