Static incubation affects early embryonic development with, notably, a reduction area vasculosa expansion and diminished sub-embryonic fluid (SEF) volume, effects produced during a ‘critical’ period (3-7 days in the chick) (Baggott et al., 2002). Also, as noted by Babiker & Baggott (1992), SEF is produced in bulk only after the appearance of the yolk sac vasculature (YSV), which undergoes extensive proliferation before and during the critical period. Quantification of such changes in YSV requires estimates of both the quantity of vessels and the degree of branching. In the chick, total vessel length increased linearly up to 160h of incubation, whereas branching was maximal by about 96 h (Vico et al., 1998); so, by the critical period branching is complete yet vessel growth continues. It would seem likely, therefore, that a lack of turning would reduce both measures of YSV proliferation during the critical period. In quail the effect of static incubation seems not to be simply due to retardation of YSV proliferation, as vascular density index was reduced in unturned eggs in the middle of the critical period, only to increase again by 168 h. Also early in the critical period fractal dimension was 1.70 (as in the chick, Vico et al., 1998), yet then decreased in unturned eggs, although not significantly, and subsequently an increase occurred. Thus during the critical period static incubation specifically affects the structuring of the YSV but whether this is because of, or independent of, retardation of area vasculosa expansion is not known
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