There are several possible causes of the absence or low frequency of many chromosomal and genetic types in crosses between 42- and 28-chromosome species of wheat. The actual effects of these possible causes are:1. Pre-gametic causes — effect small. Among pollen grains which develop normally all chromosomal types are found but not quite in theoretical proportions; those with chromosome numbers intermediate between the 14 and 21 of the parents are somewhat deficient. Intermediate numbers are more abundant among retarded grains.2. Gametic(a) Male(1) Outright abortion of pollen grains — 5 to 10%.(2) Retardation of development — 25 to 30%.(3) Failure of pollen germination — the above 25 or 35%, plus probably as much more.(4) Competition and selective fertilization — impossible to determine accurately but probably some effect.(b) Female — very little. Apparently nearly all female gametes are capable of functioning but 50% usually remain unfertilized.3. Endospermic — abortion and abnormal development cause important effects particularly in relation to female gametes but they cannot be separated completely from possible direct embryonic effects.4. Embryonic — about half the embryos abort before the seed is ripe and at least half theF2seeds usually fail to germinate. These are in part and may be wholly endospermic effects.The gametic and endospermic influences are such as to reduce the proportion of plants with chromosome combinations intermediate between those of the parents.