Three lines of recent advance relating to chromosomes and genetics are critically reviewed. The discovery that the number of chromosomes may be multiples of a basal number in several species of a genus is discussed in the light of recent observations that the number of chromosomes may be suddenly doubled in a single individual. Some of the risks encountered in interpreting all polyploid series in terms of doubling are pointed out as well as the improbability that the chromosome numbers can be permanently changed by the addition or subtraction of 1 or 2 chromosomes at a time as deVries, in particular, has recently assumed to be one of the characteristic changes that give some of the typical mutant types of Oenothera. The possibility of new types becoming established through hybridization when one of them is already a multiple of the other is made probable by the recent results of Clausen and Goodspeed in tobacco and by Ljundahl in hybrid poppies. The recent discovery of a differential pair of chromosomes in certain dioecious flowering plants is discussed. The results fall in line with the genetic evidence indicating that in plants the male is the digametic sex. It is pointed out that there is no inconsistency in the presence of such a sex-determining mechanism and the possibility of reversing the sex-characters by external agents. A discussion follows as to the somewhat paradoxical use of the term sex as applied to the haploid stages of lower plants and to the diploid stages of higher plants. The apparent contradiction is only nominal or verbal. Whether it is advantageous to apply the term sex to the conjugating hyphae of dioecious toadstools and to the conjugating swarm spores of certain algae may be open to question, and may be conceded as only a matter of taste or convenience. It is suggested that the union and failure to unite in these cases may be treated rather as problems in cross- and self-sterility which East''s recent work on tobacco has put on a good genetic basis. Finally the evidence that the Y-chromosome in certain fish and in one of the beetles may carry genetic factors is analyzed in some detail and the possible bearing of the facts on the interpretation of the sex-chromosome mechanism is pointed out.