Hematocrit and hematocrit regulation have the potential to affect developing embryos. To examine the ability of chicken embryos at day 15 to regulate hematocrit, they were subjected to either repeated saline injections (5% of total blood volume) or repeated blood removal (5% of total blood volume). Embryos showed an ability to maintain hematocrit (~20%) despite blood volume increases up to 115% of initial blood volume. Embryos were not able to maintain hematocrit in the face of dramatic blood volume loss. Oxygen consumption of embryos could be affected by their level of hematocrit. To examine this, chicken embryos at day 15, 16, and 17 of incubation were given a high hematocrit (~50-60%) sample of blood (400 μl) to artificially increase the hematocrit of the embryos (~10-12%). Despite the increase in oxygen availability, when monitored over a period of six hours, embryos showed no difference (0.36 ± 0.01 (ml O2 - min-1- egg-1) in metabolism from baseline measurements at day 15, 16 and 17.

To examine if a rhythm can be entrained in either heart rate or oxygen consumption in late stage embryos (days 17-19.5) with light as a zeitgeber, chicken embryos were incubated in complete darkness (D:D) and 12:12 light:dark cycle (L:D). Light had no impact on oxygen consumption (390 µL O2∙min-1∙egg-1) but increased heart rate for non-internally pipped embryos (260 to 270 beats∙min-1 during light cycle). Oxygen consumption increased independent of pipping while heart rate increased (255 to 265 beats∙min-1) in D:D embryos due to pipping. A light-induced rhythm or effect occurred in heart rate but not oxygen consumption, suggesting heart rate and oxygen consumption may be uncoupled.