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Whitman Laboratory of Experimental Zoology, University of Chicago, Chicago 37, Illinois

1. Experiments have been conducted which show that Xiphophorus maculatus females which have been successfully inseminated can continue to give birth to young after isolation from contact with male fishes. Although no maximum limit has been established, three to five broods of young has been shown to be the general trend.

2. Females ceased to produce young because of the lack of viable spermatozoa within the ovary. After the spermatozoa within the ovary cease to fertilize ova, the female can be inseminated again and can again produce broods of embryos.

3. By hybrid matings support has been given to the concept of isolating factors between X. helleri and X. maculatus in nature. Where attempts have been made to inseminate female X. helleri with X. maculatus spermatozoa, the results were very poor or failed completely.

4. The period of time between insemination and birth of the first brood is longer than the periods of time between subsequent broods produced from the same insemination.

5. Copulation by a female with two male fish in rapid succession failed to produce broods of embryos in these experiments.

6. The general implications of these experiments are suggested.