RESUMEN
Primo-vessels have been observed in the rat abdominal cavity as floating thread like structures on and not adhering to fascia-wrapped internal organs. To date their presence, locations, and lengths have been irregular and unpredictable, and their identification not regularly repeatable, thus they have remained a nagging enigma in primo-vascular system research for several years. In this work, locations were found where primo-vessels were regularly present and observed repeatedly. These vessels were not floating or freely movable but lay in a regular position in the mesentery in the abdominal cavity of the rat, being observed between the cecum and small intestine and between the colon and mesentery root. The difference between a lymph vessel and a primo-vessel is described in anatomical and histological aspects. In addition, trypan blue was found to enter primo-vessels through the surrounding membranes and filled spaces between fibers comprising the primo-vessels. It is conjectured that the previously observed floating primo-vessels had anomalously and irregularly emerged, for some unknown physiological reasons, from primo-vessels normally located in the fascia-like mesentery.