a, Experimental design. GFP-transgenic and non-transgenic C57BL/Ka mice were surgically joined by parabiosis such that they developed a common, anastomosed vasculature. After 6–8 months of shared circulation, pairs were treated with PMS and HCG, and analysed for blood chimaerism and for the presence of cross-engrafting oocytes. b, Predictions of the experiments. If circulating germ cells contribute to oocyte formation in adult females, then the population of ovulated oocytes collected from parabiotic mice will contain both host-derived and partner-derived cells (that is, both GFP+ and GFP- oocytes). If circulating cells do not give rise to oocytes, then all ovulated oocytes collected from parabiotic mice will exhibit the phenotype of the host (that is, GFP+ in GFP-transgenic parbionts and GFP- in their non-transgenic partners). c, Representative brightfield (left) and epifluorescence (right) images of oocytes collected from the GFP-transgenic or non-transgenic partners of pair 9 (Supplementary Table 1). Total numbers of GFP+ oocytes per total oocytes examined are summarized on the epifluorescence panels for each group (n = 4 parabiotic pairs). All oocytes from GFP-transgenic parabionts expressed GFP, whereas no oocytes from their non-transgenic partners expressed GFP.