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Rh troops as remained in the theatre when Falkenhayn's orders had been carried out were sent down the Vardar, into the midst of the Bulgarian II. Army. The Austrians diverged into Monte- negro, which was completely occupied, with some severe local fighting and much secret negotiation, by the third week in Jan. 1916. But the end of pursuit did not mean rest and reorgani- zation for the poor remnant of the Serbian army. It was impos- sible to live at the halt; and a midwinter march through the Albanian mountains, brought those whom its rigours left alive to the coast of the Adriatic. Thence, after some delay, they were transferred to Corfu, where the Western Powers provided food, equipment, clothing and stores, so mitigating a disaster that they might have prevented. (C. F. A.) SERVIA: see SERBIA. SETON-KARR, SIR HENRY (1853-1914), British big-game hunter, was born in India Feb. 5 1853. Educated at Harrow and Corpus Christi College, Oxford, he was called to the bar in 1879, but developed a taste for travel and big-game hunting which carried him all over the world. He also interested himself in state colonization and was a member of the Royal Commission on Food Supplies in Time of War. He published The Call to Arms (1900-1) and My Sporting Holidays (1904). He represented S.W. Lanes. (St. Helens) in the House of Commons from 1885 to 1906 and was knighted in 1902. He lost his life when the " Em- press of Ireland " sank in the river St. Lawrence May 29 1914.

SEX (see 24.745*). The problem of the determination of sex has in recent years been greatly elucidated. Knowledge has come from several sources. Both breeding experiments and cytological observation have severally led, as will be seen, to concordant conclusions, proving that the sex of the offspring is generally decided by one or other of the germ-cells which unite in fertilization. But though in ordinary circumstances the mode of determination is now known, there are nevertheless indications that in special cases the normal course may be altered or at least disturbed by various influences, the operation of which is not understood. The reconciliation of this latter class of evidence with the former has not yet been satisfactorily effected.

Sex Determined by Spermatozoa. Knowledge of sex-determi- nation began with the observation of Henking (1891) that in cer- tain insects the spermatozoa were of two kinds, (i) those which contained a supernumerary, unpaired, or accessory chromosome (see CYTOLOGY), now generally called the X-chromosome, and (2) those without this body. McClung (1902) first suggested that this chromosome might be a determiner of sex, and took it to be the peculiarity of the male, but Miss Stevens (1905) and E. B. Wilson (1905), to whom the development of this part of the subject is chiefly due, proved that the spermatozoa bearing the X-chromosome are in these animals destined to form females. The eggs are alike in each possessing an X, and thus the somatic or diploid cells of the daughters come to have 2 X, one received from their mother and one from their father, whereas the diploid cells of the sons have one only, received from their mother. Since the gametes of the male are of two kinds, that sex is said to be hetero-gametic, the female being homo-gamelic. Further obser- vation, however, showed that the organization of even nearly allied genera of insects is by no means uniform in respect of the sex-chromosomes. Though unpaired in the males of some genera, the X may in others have a pair or " mate" of smaller size, known as the Y-chromosome. Between these and other genera in which the male has a pair of sex-chromosomes not visibly different from each other there are several transitional conditions. In a considerable number of forms also, the X is represented by a group of separate chromosomes, regarded as collectively the mates of the Y.

The X-chromosome has been seen in several orders of insects, especially Hemiptera, Orthoptera and Coleoptera, in spiders, myriapods and some nematodes. In man it is said (Guyer) to be represented by a pair corresponding to a single Y-chromosome.

Sex Determined by Ova. Naturally this discovery that the male is hetero-gametic was at first supposed to be of universal application, but the next advance, which resulted from experi- mental breeding, showed that this simple view could not be enter-

tained. The Currant Moth (Abraxas grossulariata) has a variety lacticolor, characterized by a deficiency of black pigment, till then known only in the female. Doncaster, instituting experiments with this variety, found that by breeding such females with nor- mal males the Fi family consisted of males and females, all normal. 1 Interbred, these gave Ft composed of two normal males: one normal female: one lacticolor female. But lacticolor 9 XFicf produced families containing normal o", normal 9, lacti- color cf and lacticolor 9 , all in equal numbers. On breeding any normal grossulariata 9 with the lacticolor & now produced, the sons were all grossulariata and the daughters all lacticolor.

Other interpretations have been proposed, but it is evident that the eggs of the grossulariata 9 are of two kinds, (i) those which are destined to be females, and do not carry the grossu- lariata factor, (2) those which do carry this factor, and are des- tined to be males (Bateson and Punnett). Taking the factors G, grossulariata: g, its absence; F, femaleness: /, its absence, the results may be represented symbolically thus: I. Lact. 9 XGrosscf

F/:gg I ff.Gg

r

F Gross 9

F/:Gg

gametes Fg ; /G

X

Gross cf Jf-Gg /G;/g

I

I Gross 9 F/;Gg

i Lact. 9 F/:gg

2 Grossed

I I

Lact. 9 Gross 9 F/.gg Ff.Gg Gross 9

Lact. 9 X transmitting Gross o* F/.gg I ff.Gg

X

I I

Lact. c? Gross c Jf.gg ff.Gg Lact.c?

gametes Fg;/G

Lact. 9s Gross cTs

F/:gg /.Gg

Sex- Limitation and Sex-Linkage. Such a system of heredity is sometimes called " sex-limited," and the descent of the charac- ter so limited serves as an indication of the mode by which the factor determining sex is transmitted from parent to offspring. The proof that in these moths sex is determined by the eggs, or in other words that the female is the hetero-gametic sex, is thus complete. The same has been shown to be true of birds. In fowls, several conditions have been shown to be sex-limited to females; e.g. black as against the cuckoo-markings of the Plymouth Rock; golden (as in Sebrights or Hamburgs) as against silver; the black pigment of Silky fowls is suppressed by a sex-limited factor -which inhibits this development, etc. In canaries the peculiar form of albinism known as Cinnamon, and in doves the pale albinotic variety shows a similar behaviour. We have thus to recognize that, paradoxical as it may appear, sex is in some animals determined by the sperm, and in others by the eggs. Man belongs to the former class. Apart from the cyto- logical evidence, as yet unconfirmed, the descent of sex-limited conditions, notably colour-blindness, demonstrates this. Substi- tuting male for female, colour-blindness, is transmitted in man exactly as the lacticolor character is in the moths. A colour-blind man mated with a normal woman has sons and daughters with normal colour-vision. The sons cannot transmit the colour- blindness, but the daughters transmit it to half their sons who are therefore colour-blind. Moreover, when the transmitting female is mated with the colour-blind man, the colour-blind fe- male is produced, just as in the corresponding mating (3) in the grossulariata experiment, the lacticolor male was formed. Finally when the colour-blind woman mates with a normal man the sons are all colour-blind and the daughters are all transmitters. Following Doncaster's notation in the grossulariata scheme, if M, maleness, be substituted for F, and N, normal colour-vision, for G, the same analysis represents the observed facts: the transmit-

1 In genetics the following symbols are frequently used :9 = female; cf =male; X = mated with; FI, F 2, etc. = the first filial family, the second, etc.

" These figures indicate the volume and page number of the previous article.