Some may be individual red blood cells swollen due to osmotic pressure. Others may be chains of red blood cells stuck together; diffraction patterns can be seen around these. Others may be “coagula of the proteins of the vitreous gel, to embryonic remnants, or the condensation round the walls of Cloquet’s canal” that exist in pockets of liquid within the vitreous.
For anyone too lazy to look:
~ https://en.m.wikipedia.org/wiki/Entoptic_phenomenon