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B. P. SETCHELL and N. T. HINKS

In the intermingled arteries and veins of the retia mirabilia leading to the swim-bladders of fishes (see Enns, Douglas & Scholander, 1967; Berg & Steen, 1968) O2 and CO2 are transferred from one vessel to another. It has also been suggested that O2 may pass between the arterial and venous blood in the spermatic cord (Cross & Silver, 1962) but the similar O2 saturation of haemoglobin in blood from the femoral and testicular arteries of rams did not support this idea (Setchell & Waites, 1964). However, in most marsupials, the internal spermatic artery breaks up into many branches in the spermatic cord and these branches are interspersed among a similar number of veins (Harrison, 1948, 1949; Barnett & Brazenor, 1958). Countercurrent exchange of gases would be much more likely to occur with this anatomy than in the eutherian mammals in which the internal spermatic artery
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B. P. SETCHELL, J. K. VOGLMAYR and N. T. HINKS

Summary.

Fluid was collected from the cannulated rete testis of six conscious Merino rams whose testes were heated to 40·5° C for 3 hr. Rate of fluid flow dropped during heating but recovered thereafter; there were 100- to 10,000-fold decreases in sperm concentration which appeared to be biphasic. The timing supported the suggestion that heating had affected pachytene spermatocytes and division of type-B spermatogonia.

The concentration of sodium, potassium, lactic acid, chloride, protein and testosterone in the fluid was unrelated to the number of spermatozoa present but above 20×106 spermatozoa/ml the concentration of inositol and glutamic acid appeared to be positively correlated with sperm number. Below this sperm concentration, small amounts of glucose appeared in some samples of fluid.

The results clearly indicate that the secretion of rete testis fluid can be independent of the release of spermatozoa from the germinal epithelium.