ting female (Nnmm) in man, for example, corresponds with the male Ggff, etc. in grossulariata.
There is thus no doubt that just as in the bird and the moth the female produces two kinds of eggs destined respectively to form females and males, so in man the male produces two kinds of sperms destined to form respectively males and females.
Cytological Interpretation. The notation given above provides the simplest representation of the empirically observed facts without any attempt to refer them to cytological phenomena. This attractive branch of Genetics has been actively developed by T. H. Morgan and his colleagues with remarkable success; and though serious difficulties remain, the incidence of the sex- limited characteristics can so readily be interpreted as depending on the distribution of sex-chromosomes, observed or hypotheti- cal, that a causative influence has with great plausibility been attributed to them. Starting from the now familiar fact that in certain animals the XY male has visibly one X-chromosome and the XX female two, it is argued that the " double dose " of X is the cause of the female characteristics and that one dose of the same element produces the male attributes. If, then, in such animals the dominant factors, which show linkage, be supposed to be also carried in the X-chromosome, the sex-limited distribu- tion of the negative characteristics to males will result. At first sight the fact that in other animals the female is hetero-gametic seems irreconcilable with this scheme, but, by making the assump- tion that in these females a similar cytological apparatus exists, the genetic observations may be represented on the same plan as that adopted for the hetero-gametic males. For the hetero- gametic female may be represented as XY and the male as XX, and here again if the X carries the positive element, say the G of grossulariata, then the gamete of the composition XG is always destined to the sons, and Yg to the daughters, as the facts re- quire. Perhaps the strongest evidence in favour of the chromo- some hypothesis has been found in the phenomenon of "non- disjunction " in Drosophila, the fruit-fly, whose genetical composi- tion has been studied more fully than that of any other organism (see GENETICS). In this creature sex-limitation is to males, and the male is hetero-gametic; but, exceptionally, unexpected forms appear, as, for instance, red-eyed males in the mating where white-eyed alone should normally be produced. Bridges found that in such individuals and among the families containing these abnormal members, irregularities in the occurrence of the chromosomes could be demonstrated. Apparently in some maturation division two X-chromosomes had passed to the same egg, others receiving none. From this a series of exceptionally constituted gametes are derived which could have brought about the observed exceptions. Another line of argument pointing to the same conclusion has been derived by Morgan and his asso- ciates from a study of gynandromorphs the curious individuals composed of more or less irregular patchwork of male and female tissues, which are formed with some frequency especially among the various orders of insects. A number of these have occurred in the pedigreed families of Drosophila, and on analysis it was found possible in almost every case to refer the characters shown in the several parts to their parental origin.
A primary objection to these modes of interpretation is that the cytological conclusions rest on a still slender basis of observa- tion. As regards the forms with hetero-gametic females an unpaired chromosome has been reported to exist in the females of two moths (Phragmatobia and a Psychid), but on the other hand, Guyer states that in certain birds the male has an unpaired chromosome. Commonly, moreover, no consistent cytological distinction between the two sexes can be observed. But a still more fundamental difficulty besets the conception of the charac- teristics of either sex as dependent on a merely quantitative distinction. From arguments which cannot be here developed the Y-chromosome is regarded as demonstrably empty, and containing no determining elements, and XY is therefore to be understood as in potentiality merely the half of XX. Chemical phenomena, vaguely analogous, have been appealed to as making this system of interpretation less inadmissible than it at first appears, but that it is beset by grave objections cannot be denied.
Cytology has been of great assistance in codifying the ap- parently contradictory records as to the intricate series of parthenogenetic or agamic and sexual forms of the Hymenoptera and Hemiptera. We have learnt for example that in Hemiptera (Morgan; von Baehr) when fertilization produces females only, the fact is due to the death of all the sperms, which do not bear the X-chromosome. But in these groups it not rarely happens (Neuroterus, Doncaster) that the fertilized females give rise parthenogenetically to other females of which, without fertiliza- tion, some produce females only and others males only, but no cytological distinction between these two types of females has been seen. Whatever be the true cytological account of sex- determination, we have nevertheless to recognize that femaleness and maleness respectively, though similar throughout the Meta- zoa both in outward manifestations and in deeper physiological features, are constructed upon at least two distinct genetic plans. To reconcile the infrequency of visible cytological distinctions between the sexes with the chromosome hypothesis it has been suggested that the critical bodies are in such cases attached to other chromosomes, but this, though undeniable as a possibility, is as an argument dangerously tinged with obscurantism, a comment which applies to many of the subordinate hypotheses supporting the chromosome theory of genetic causation.
The situation is one through which many scientific problems have passed before final solution has been obtained. By independ- ent lines of evidence conclusions largely identical have been reached. The facts are not in dispute, but a consistent interpre- tation of the whole series has not at present been obtained.
Disturbance of Normal Sex-Ratios: the Production of Inter- Sexes. Hybridization is not rarely followed by a disturbance of the normal sex-ratio. From canary females crossed with other finches and from domestic hens crossed with cock-pheasants male mules (sterile) are easily bred but female mules seldom if ever. Among Bovidae some crosses give fertile females but sterile males (S. v. Nathusius) and the same occurs in a cross between species of cavies (Detlefsen). Among Lepidoptera a series of such ex- amples are known, the first having been observed by Standfuss, who called attention to the absence or scarcity of females in many species-crosses. Recently Goldschmidt, working with races or species of Lymantria dispar brought together from many coun- tries, carried out a remarkable series of experiments. He found that a Japanese race used as 9 X European cf gave normal males and females, but that in the reciprocal cross though the males came normal, the females were intcrsexes, exhibiting many transitional stages, approaching males in various degrees both in form, colour and instincts. When Japanese males of certain races were used, the whole of the female offspring are said to have been thus transformed into males. Goldschmidt states that the various races can be arranged in an ascending scale according to the completeness with which these effects are produced, but the phenomena showed many complications not as yet adequately represented, notably the appearance of par- tially transformed male intersexes in the p2 generation raised by inter-breeding the normal offspring raised in the first experi- ment. It has been pointed out by J. B. S. Haldane (1920, unpub- lished) that, apart from certain exceptions, a general principle can be perceived in this group of phenomena. If in hybrid off- spring either sex is consistently missing or defective, this will be the hetero-gametic sex, e.g. in birds and Lepidoptera the female, in mammals, the male.
Sex of Twins. The facts hitherto dealt with all go to prove that sex is determined by the gametic contribution made by one or other of the parents and that this is the normal course is not open to question. In harmony with that conception of sex- determination, it is found that when, as in twinning of embryos and generally when the fertilized egg multiplies by division, the products are commonly all of the same sex. In the armadillo (Tatusia novem-cincla), studied in detail by Newman, four em- bryos, demonstrably arising by division of one blastodermic vesicle (developing ovum), are usually produced at a birth and are always of the same sex, the four being all males or all females. Patterson found an exception in the case of the parasitic
