the other, the possible number of combinations in the production of a new individual from such germ-cells would be 8x8, or 64. With the larger numbers of chromosomes which exist in most animals it is readily seen that the number of possible combinations becomes very great. Thus any individual of a species with twenty chromosomes--and many animals, including man, have more--would have ten pairs at the reduction period and could therefore form (2)^{10}, or 1,024 different gametes in each sex. And since any one of these in one sex would have an equal chance of meeting with any one in the opposite sex, the total number of possible different zygotes that might be produced would be (1,024)^{2}, or 1,048,576. Sex therefore, through recombinations of ancestral materials, undoubtedly means, among other things, the production of great diversity in offspring. DETERMINATION OF SEX =Many Theories.--=From earliest times the problem of sex determination has been one of keen interest, and needless to say hundreds of theories have been propounded to explain it. Geddes and Thomson say that Drelincourt recorded two hundred sixty-two so-called theories of sex production and remark that since his time the number has at least been doubled. The desirability of controlling sex has naturally appealed strongly to breeders of domesticated animals. A study of animals born in litters, or of twins, is enough in itself to make us skeptical of theories of sex-determination based on nutritional or external factors. In a litter of puppies, for example, there are usually both males and females, although in their prenatal existence they have all been subject to the same nutritional and environmental conditions. Likewise in ordinary human twins one may be a boy, the other a girl, whereas if the nutritional condition of the mother were the fact determining sex, both should be boys or both girls. However, there are twins known as _identical twins_ who are remarkably alike and who are always of the same sex. But there is reason to suppose that identical twins in reality come from the same zygote. Presumably in early embryogeny, probably at the two-celled stage of cleavage, the two blastomeres become separated and each gives rise to a complete individual instead of only the half of one it would have produced had the two blastomeres remained together. Such twins are monochorial; that is, they grow inside the same fetal membrane, whereas each ordinary twin has its own fetal membrane and has obviously originated from a separate ovum. It has been established experimentally in several kinds of animals that early cleavage blastomeres when isolated can each develop into a complete individual. In man, ordinary twins are no more alike than ordinary brothers and sisters, but identical twins are strikingly similar in structure, appearance, habits, tastes, and even susceptibility to various maladies. The fact that they are invariably of the same sex is a strong