Fig. 8. (A) Steps leading to assembly of the trunk angiogenic vascular network and (B) a proposed model for determination of secondary sprout fate and intersegmental vessel AV identity. For clarity, both diagrams show the vessels on only one side of the trunk. (A, part i) Primary sprouts emerge bilaterally exclusively from the dorsal aorta (red). (A, part ii) Primary sprouts grow dorsally, branching cranially and caudally at the level of the dorsolateral roof of the neural tube. (A, part iii) Branches interconnect on either side of the trunk to form two dorsal longitudinal anastomitic vessels (DLAV). (A, part iv) Secondary sprouts begin to emerge, exclusively from the posterior cardinal vein (blue). (A, part v) Some secondary sprouts connect to the base of primary segments, while others do not. (A, part vi) Primary segments with patent connections to secondary segments become intersegmental veins (blue), while primary segments that remain connected only to the dorsal aorta become intersegmental arteries (red). Most of the secondary sprouts that do not connect to primary segments serve instead as ventral roots for the parachordal vessels. Intersegmental veins form additional connections to the parachordal vessels at the level of the horizontal myoseptum. (B) How flow dynamics might help to guide the patterning of vessel connections (see Discussion for details). Primary segments without blood flow are shown in gray, while those carrying arterial or venous blood flow are shown in red and blue, respectively. Unconnected (growing) secondary sprouts are shown in black. Flow through a primary segment inhibits connection to the segment by an adjacent secondary sprout (inhibitory cues are shown as sideways `T' symbols).