 The VA-A (purple) appears as an isolated, etv2-expressing, clo/npas4l-dependent angioblast in the anterodorsal yolk sac region at 29 hpf, before migrating ventrally towards the VA and expressing fli1 and kdrl by 34 hpf, and lyve1b at 36 hpf. Around this time, facial lymphatic development in the head begins as the FLS (green) arises from the dorsolateral region of the CCV, while lymphatic progenitors become specified in the PHS (orange and yellow cells). At 42 hpf, the dorsal portion of the VA-A differentiates into the VA-L (pink) as it acquires a lymphatic cell fate by turning on Prox1 expression, while the ventral portion of the VA-A remains Prox1-negative (purple). Both the VA-L and the FLS begin migrating dorsally and anteriorly, respectively, towards the PHS, which at this time contains the PHS-LP (orange and yellow) domains. By 54 hpf, the VA-L has reached the FLS tip, which now consists of the PHS-Ls (orange and yellow) that have sprouted from the PHS and fused to the FLS. By 72 hpf, the VA-L has fused to the FLS to form the LFL, while the ventral Prox1-negative VA-A (purple) fuses to its contralateral counterpart and to the first aortic arch to form the HA. At 120 hpf, all components of the facial lymphatic network are present, with the OLV, MFL and LAAs sprouting from the existing LFL 