CXCL12 and hydrops fetalis: Therefore, we think that the observed difference is most parsimoniously explained by a strong, postnatally maintained, chemoattractant influence of the chemokine CXCL12 (produced by cells investing the HF; Lu et al., 2002; Anstötz et al., 2019) on CXCR4-expressing Cajal–Retzius cells (Stumm et al., 2002; Marchionni et al., 2010).