The circulating erythrocytes of rainbow trout ( Salmo gairdneri)

1. 1. Washed erythrocytes from rainbow trout maintained nonnoxically at near-optimum temperature (14°C) on a high nutritional plane and seasonally-neutral photoperiod (12L:12D) with minimum disturbance were separated by velocity sedimentation at unit gravity into 10 fractions. 2. 2. On the basis of constancy of mean cell hemoglobin content, 81.9% of circulating red cells were judged to be mature. 3. 3. What may have been postmature cells accounted for a further 7.5% of the total. The remainder, 10.6%, were presumed to be juvenile. 4. 4. The 11-membered hemoglobin isomorph system exhibited significant ontogenetic variations in the abundances of several minor, but no major isomorphs. 5. 5. Ultrastructural examination confirmed that cell elongation during maturation is correlated with development of the marginal band microtubule system. 6. 6. Hemoglobin formation appeared to be sequentially correlated with the appearance and complexity of the segregation apparatus, and the abundances of mitochondria and polysomes. 7. 7. The question of postmaturational hemoglobin synthesis was not resolved.