Melanophores are also present on the dorsal and lateral surfaces of the yolk sac. Beginning at this time it be useful to examine the embryo with epi-illumination, using either the dissecting or compound microscope, not just with transmitted light. This is because the first reflective pigment cells, iridophores, soon appear on the eye. However, one cannot identify them at 36 h. The pericardial cavity is not yet prominently inflated.
Blood circulation is strong.
The heart, previously a straight tube, now bends slightly. There is still but a single pair of aortic arches. Returning blood through the trunk and across the yolk, the pathway that includes the axial vein, the posterior cardinal vein and common cardinal vein are well-defined channels. Spontaneous episodes of lashing activity of the embryo are now infrequent, occurring at less than one episode per min. The reflexive escape response to a touch lasts longer than before, and now effectively serves to displace the embryo, usually up to several body-lengths.