November 6, 1919] 



NATURE 



215 



not easy. Much of the new work is in an in- 

 cipient stage, an3 that which is the most attrac- 

 tive of all — namely, Morgan's effort to establish 

 a close connection between cytological appear- 

 ances and the results of experimental breeding — 

 promising though it is, must be tried by tests on 

 a scale far wider than experience of Drosophila 

 provides before we are able to assess its value 

 with confidence. Whether the theory that the 

 factors are arranged in the chromosomes, like 

 beads on a thread, stand or fall, it has already 

 served the purpose of a good theory. It has 

 fired the minds of many workers, and has directed 

 their inquiries with manifest success. Its weak- 

 ness lies first in the narrowness of the field 

 studied, but besides this it is not yet wholly free 

 from the objection that the subordinate and inci- 

 dental hypotheses are not altogether independent 

 of each other. 



Various as are the methods of attack, the 

 objects before us are sufficiently clear. Among 

 them the most important is a determination of 

 the moment or moments at which segregation 

 may occur. To the solution of this problem 

 most of the investigations contribute. On one 

 hand, we have the large body of facts consistent 

 with Morgan's view that synapsis is the critical 

 moment. Were our outlook confined to animals, 

 we should scarcely hesitate to accept that hypo- 

 thesis as satisfying the conditions, but the plants 

 give no such clear answer. Not only is an obvious 

 somatic segregation leading to genetic diversity 

 of the parts not rare, as in many variegated 

 plants and plants which give dissimilar forms 

 from adventitious buds, but there is now a large 

 group in which the male and female organs of 

 the same plant differ in the factors which they 

 carry. Miss Saunders's stocks are the classical 

 example, where the male side carries doubleness 

 and cream plastid colour, whereas the ovules are 

 mixed in these potentialities. Similar sex-linkage, 

 as, following Miss Pellew's use, it may provision- 

 ally be called, has been shown to exist in Petunia, 

 Campanula carpatica, Begonia Davisii, and in 

 certain forms of CEnothera. 



In all such examples segregation cannot be 

 supposed to occur later than the constitution of 

 the sexual organs. Collins 's experiment, show- 

 ing that in Funaria the scales surrounding the 

 male organs by their vegetative growth give rise 

 exclusively to male mosses, is another and very 

 striking indication to the same effect. The 

 genetics of " rogue " peas point to a similar con- 

 clusion in regard to the distinction between the 

 rogues and the type from which they come. In 

 some way not yet clear, the type-elements are 

 wholly or partially excluded from the germ- 

 lineage of the heterozygotes, being apparently 

 relegated to the lower parts of the stem. Such 

 facts raise a suspicion that, considered as genetic 

 machines, plants may be fundamentally distinct 

 from animals, an idea already suggested by the 

 contrast between their modes of growth. In ihe 

 animal the rudiments of the gametes are often 

 visibly separated at an early embryonic stage, 

 NO. 2610, VOL. 104] 



whereas in the plant they are given off from 

 persistent growing points. Indeed, since Baur's 

 work with variegated chimaeras, which led to his 

 brilliant interpretation of Winkler's "graft- 

 hybrids," this possibility has inevitably been 

 present to our minds. 



In knowledge of the nature of sexual difference 

 many very substantial advances have been made, 

 j which have much extended the original discovery 

 I that sex depends on a segregating Mendelian 

 j factor, in some forms the male, in others the 

 female being the heterozygous member. In the 

 fowl femaleness is dominant, and the hen is 

 heterozygous in sex, from which Morgan drew 

 the interesting corollary that the "henny" char- 

 acter of the Sebright cock is also a dominant. 

 Not only has this been proved experimentally, 

 but he has lately shown that after castration the 

 Sebright cock acquires ordinary cock's plumage, 

 much as hens do in ovarian disease. Perhaps we 

 may regard the henny male as containing part of 

 the large compound factor which normally con- 

 stitutes femaleness. Conversely, we may inter- 

 pret the spurs frequently present in normal Leg- 

 horn hens as indicating that they have lost that 

 part of the female factor which inhibits the growth 

 of the spur. Whether such transference involves 

 actual detachment of chromosome material, as 

 Morgan's theory would demand, is uncertain. 

 Nevertheless, an approach to such evidence is 

 I provided by the extraordinarily interesting 

 observation of Bridges of a condition which he 

 calls non-disjunction. Certain crosses in Droso- 

 phila failed to exhibit the normal sex-limitation, 

 ! and unexpected terms appeared. Bridges was 

 able to show that in the families which behaved 

 in this way an extra sex-chromosome sometimes 

 occurred, carried over, as he imagines, by some 

 error of division. Not improbably Doncaster's 

 ■ female-producing strains of Abraxas grossulari- 

 1 ata, in which evidence of an extra chromosome 

 was found, are an analogous case. Patterson 

 with great probability proposes a similar explana- 

 tion for the curious phenomenon which he has 

 investigated in Copidosoma, where, by poly- 

 embryonic division of a single egg (almost cer- 

 tainly), males, females, and inter-sexes may 

 result. The inter-sexes seen by Kuttner in 

 Daphnia, and those produced by J. W. Harrison 

 with considerable regularity in some hybrid com- 

 binations of species of Geometers, are obviously 

 to be considered in this connection, and doubtless 

 the sterile males, accompanied by fertile females, 

 which Detlefsen found as the normal produce of 

 a species cross in Cavia, will be investigated with 

 such possibilities in view. 



But though sex behaves in so many ways as 

 a Mendelian allelomorph, showing, of course, 

 frequent phenomena of linkage, it begins to be re- 

 markable that no case of crossing-over in respect 

 of these linkages has yet been established. Were 

 the sex -chromosome always mateless, this fact 

 would fit admirably with Morgan's views, but 

 since the %-chromosome not rarely has a mate, 

 a distinct problem is created. As bearing on the 



