Chromosome numbers were determined in numerous male gametophytes of F1 between 21- and 14-chromosome species of wheat. The results show that pollen grains with various chromosome numbers from 14 to 21 are actually formed and in approximately the theoretically expected proportions. The lack of plants in later generations which should result from the functioning of pollen grains with intermediate numbers is therefore not due to the failure of such grains to be formed because of a lack of random segregation at the second reduction division.Grains with intermediate numbers are retarded in their nuclear development, so that counts made on stamens in which division is most active give a smaller proportion of grains with intermediate numbers and a higher proportion with parental numbers than is expected theoretically. Retardation in nuclear development is correlated with a deficiency in cytoplasmic contents, 10 to 15% of the grains showing little or no cytoplasm, and another 15 or 20% showing some degree of reduction in cytoplasm. AH grains with reduced cytoplasm and some of those with normal contents are so retarded in nuclear development (having only one or two nuclei or no organized male cells) that they could not function when the normal ones are mature and the stamen dehisces. Unfavorable chromosome conditions in grains with intermediate numbers cause a complete abortion of some grains and retardation of nuclear development in others.Under the best available experimental conditions only 11 or 12% of F1 pollen grains germinate, in contrast to 70 or 80% for parental pollen. No grains with reduced cytoplasm germinate, and at least 50% of those with apparently normal cytoplasm fail to germinate.