In addition to the mandibular arch (AA1), a vestigial hyoid arch (AA2) appears on either side [1.3 dpf head, lateral]. The radix of the LDA, where the right and left LDA merge together to form a single midline dorsal aorta (DA) which continues into the trunk, can be clearly seen [1.5 dpf head, dorso-lateral]. This junction point occurs at approximately the same A-P level where the ACV and PCV merge together laterally to either side into the CCV (or "duct of Cuvier," DC). The right and left CaDI extend further caudalward from the BCA as posterior communicating segments (PCS) along the base of the midbrain, and merge to the single basilar artery (BA, [1.5 dpf head, dorso-lateral]). Although the circle of vessels comprised of the BCA and PCS resembles the so-called Circle of Willis of humans, these are not homologous to one another. The BCA does not correspond to the anterior communicating artery of the mammalian Circle of Willis, which connects the anterior cerebral arteries. The metencephalic arteries (MtA) branch dorsally from both sides of the PCS, looping up to join with the opposite MtA as well as with both MCeV at the dorsal midline of the head (dorsal midline junction, DMJ; [1.5 dpf head, dorso-lateral]). The mesencephalic veins (MsV) sprout rostrally from this junction point as well. The BA extends caudally along the base of the medulla oblongata. Initially this vessel is unbranched but by late 1.5 dpf capillaries have begun to form connecting the BA to the two PHBC running lateral and parallel to the BA on the ventral surface of the medulla oblongata [1.5 dpf head, dorso-lateral]. The BA does not continue into the trunk at this stage but terminates near the end of the medulla oblongata.

The optic artery (OA) branches from the PICA at 1-1.2 dpf and enters the eye ventrally through the optic fissure as the hyaloid artery [1.5 dpf head, ventral-anterior-lateral]. Initially it then forms a single loop and exits the optic fissure as the hyaloid vein, continuing to the PHBC as the optic vein by 1.3 dpf. The portions of the optic vessels internal to the eye are called the hyaloid vessels, while the external portions are called the optic vessels. A new vessel, the primitive prosencephalic artery (PPrA), branches from the CrDI shortly after it exits the PICA. It extends laterally around the rostral end of the head, looping around to link to the equivalent PPrA’ vessel from the other side of the embryo, and to both ACeVs, at the rostral cranial midline. The root of the PPrA becomes disconnected from the CrDI very shortly after it comes on line [1.3 dpf head, lateral] and [1.5 dpf head, ventral-anterior-lateral]. At or slightly after this time the PPrA, now called the prosencephalic artery (PrA), becomes instead connected to and fed by the anterior mesencephalic central artery (AMCtA) and also by the palatocerebral artery (PLA), via a communicating vessel (CMV). The AMCtA and PLA are described further below (see the "2 day" description in the next section). Another new vessel, the primitive mesencephalic artery (PMsA), branches dorsally from the CrDI, then curves caudally along the dorsal-medial wall of the eye capsule to drain into the PMBC just rostral to the midbrain-hindbrain boundary [1.3 dpf head, lateral]. At later stages of development the PMsA becomes disconnected from the PMBC and drains via an alternate route. The remaining part of PMsA will be used by midbrain central arteries as their drainage into the PMBC-PHBC junction (see 2 dpf description). Additional new branches from the CrDI, the palatocerebral artery (PLA) and the nasal ciliary artery (NCA), appear somewhat later, at 1.5-2 dpf [1.5 dpf head, ventral-anterior-lateral]. The NCA bifurcates as it almost reaches the lens in the next stage [1.5 dpf head, ventral-anterior-lateral],[2 dpf head, ventral-anterior-lateral],[2 dpf head, dorsal-lateral]. The dorsal branch from the NCA becomes the dorsal ciliary vein (DCV) and drains into the PMBC or the PMBC-PHBC junction. The ventral branch from the NCA empties via the optic vein. Portions of both of these NCA branches are later part of the completed inner optic circle (IOC; for example see [2.5 dpf overview]). The PLA branches off ventrally from the CrDI shortly after it exits the PICA [1.5 dpf head, ventral-anterior-lateral]. It extends rostrally along the base of mid and forebrain, then loops to meet the PLA’ from the other side of the embryo at the "PLA junction" (PLAJ; see [2 dpf head, ventral-anterior-lateral] and 2 dpf text for further information). The "wiring pattern" of the cranial vasculature and changes that occur in this pattern between 1.5 and 2 dpf are diagrammed in [1-2 dpf head vessel changes].