Direct Observation of the Transfer of Pronuclei in Living Conjugants of Paramecium bursaria

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Abstract
The conjugation process in para-mecia was studied in living animals to analyze nuclear and cytoplasmic behavior and determine the time relationships involved. Two races of P. bursaria, B9 and 255, were used. The organisms show a feeble mating reaction at 4:45 A.M. at 25[degree]C and this increases in strength until at 8 A.M. it is strong. After 1:30 P.M. it decreases, being feeble at 4:30 P.M. In conjugation the 3 pregamic divisions occur at 25[degree]C. The 1st division is completed about 14 hrs. after mating, the 2d 1 hr. after the first, and the 3d division, involving exchange of pronuclei and syngamy, 16-18 hrs. after mating. The micronuclear product is the one destined to undergo a succeeding division. The one farthest from the paroral region degenerates. The dividing spindle of the 3d pregamic division arranges itself at nearly right angles to the long axis of the conjugant and presses against the membrane at the point of crossing. It takes about 20 min. for the migratory pole of the spindle to pass from one conjugant to the other. Cyclosis in each conjugant is slow, and crystals are bunched around the dividing spindles. After syngamy, the 1st amphinuclear division is completed about 20 hrs. after mating, the 2d, 22 hrs., and the 3d, 24 hrs. Conjugants may be seen completing the 3d amphinuclear division at the time of separation 24 hrs. after mating.