In certain forms parthenogenetic and sexual reproduction may alternate in a cycle, _e. g._, in plant lice, _Daphnia_, and rotifers. In plant lice it has been observed for a long time that when the plant is normal and the weather warm the aphides remain wingless, reproduce parthenogenetically, and only females exist, and this may last for years and for more than fifty generations; but that when the plant is allowed to dry out both sexes appear. Here we are dealing with a limited determination of sex inasmuch as the experimenter has it in his power to prevent or allow the production of males. The facts do not in all probability contradict the statements made concerning the rôle of the X chromosomes in the determination of sex. We have seen that where sex is determined by two types of spermatozoa one type of eggs is produced which possesses only one X chromosome. Such eggs might produce males if not fertilized (as they do in bees), but they cannot produce females because for that purpose they must have two X chromosomes. It has been shown for certain cases, and it may be true generally, that if eggs of this type give rise to parthenogenetic females they may do so because they have for some reason two X chromosomes. Usually such an egg loses one of the X chromosomes in a process of nuclear division (the so-called reduction division) which usually precedes fertilization. If this reduction division is omitted the egg has two X chromosomes and if such an egg develops parthenogenetically it gives rise to a female. These cases do not, therefore, contradict the connection between X chromosomes and sex determination established by cytological observations and breeding experiments, on the contrary, they confirm it. The question remains: How can external conditions bring it about that the reduction division is omitted? To this question no definite answer can be given at present. We may in passing mention the well-known observation that twins which originate from the same egg always have the same sex; while twins arising from different eggs show the usual variation as to sex. Twins coming from one egg have the same chorion and can thereby be diagnosed as such. They can be produced as we have stated in Chapter V by a separation of the first two cleavage cells of the egg, each one giving rise to a full embryo. It harmonizes with all that has been said above that the sex of two such individuals must be the same since they have the same number of X chromosomes, the latter being determined in the human race by the nature of the spermatozoön which enters the egg.