238 



DISCOVERY 



sexes, but not directly concerned with the reproduc- 

 tive organs? Here again, recent research has given 

 us a definite answer. The factors necessary for the 

 development of the characters of both sexes are 

 present in all individuals of any of the higher animals, 

 but normallv onlv those which are proper to one sex 

 actually develop. The presence of one or of two 

 X's acts like a switch, which alters the condition in 

 the developing embryo in such a way that in one case 

 only the male characters can reveal themselves, in the 

 other case the female characters. A female animal 

 contains, locked within the chromosomes of her cells, 

 the factors which in other circumstances could com- 

 bine to build a male, the male contains, never 

 expressed in reality, the constitution of a female. 

 That this is so is shown definitely bv the facts 

 observed when two species are crossed. The males 

 of difterent species of pheasants, for instance, differ 

 from each other very much in plumage, while the 

 females are all more or less drab and alike. If :t 

 female Reeves pheasant is crossed with a male of 

 another race, the males among the hybrid offspring 

 show many characters of the male Reeves pheasant, 



Fig. i.—.\ feminized M.\LE GUIXE.\ PIG .ACTING .-VS SUCKLING 

 FOSTER-.MOTHER TO A YOU.NG SPECIMEN. 



By permission of the Editor of the ^'Journal of the Royal Society of Arts." 

 {After Steiuach.) 



although these characters must have been transmitted 

 through the chromosomes of the mother, in whom 

 they were invisible. 



In in.sects, the simple presence of one or two X's 

 in the cells of the body is enough to call forth the 

 proper sex-characters; but in higher animals like birds 

 and mammals, there is another link in the chain. 

 This link is furnished bv the reproductive organs. 

 The sex-chromosome machinery acts as a switch 

 which allows either male or female reproductive 

 organs to develop in the embrvo ; but as soon as 

 these are developed, they start producing a .secretion 

 or hormone which is necessary for the development of 

 all other sexual characters. The most complete proof 

 of this has been afforded by the extraordinary experi- 

 ments of Steinach, .Sand, Moore and others, who have 

 removed the reprf)ductive organs from voting rats or 

 guinea-pigs, and grafted into them reproductive 

 organs taken from individuals of the opposite sex. 



The result has been a complete alteration in the 

 animals' growth, ending in an almost complete 

 assumption of the characters of the opposite sex. 

 -Male guinea-pigs whose reproductive organs have 

 been removed and replaced by ovaries have even 

 vielded milk and suckled voung in the normal way 

 (see Fig-, i), and show the sexual Instincts proper to 

 females. Such animals can, of course, not breed, 

 since at the time of the operation, the internal organs 

 associated with reproduction were already laid down, 

 and only the subsequent growth of the animals was 

 affected. Recently, however, ingenious experiments 

 have been carried out bv which developing hens' eggs 

 were opened, and a small piece of reproductive organ 

 from a fowl grafted on to the membrane surrounding 

 the yoke, ^\'hen the operation was successful, and 

 the chick was of opposite sex to the reproductive 

 organ grafted on to It, the whole development of its 

 reproductive system was aftected, and the chicks 

 reached the stage of hatching in a condition Inter- 

 mediate between male and female, often nearly trans- 

 formed Into the opposite sex from that which they 

 by rights should have been. 



Very interesting results have also been obtained on 

 adult birds. Here it is found that the ovary secretes 

 some substance which prevents the development of 

 male plumage. .\ capon, or any male bird with 

 reproductive organs removed, shows no alteration of 

 plumage. But a hen bird whose ovaries are taken 

 out \\ill at the next moult assume the plumage proper 

 to the male. Further, there exist certain breeds of 

 fowls, such as the Sebright bantam, in which the cocks 

 are hen-feathered, and possess none of the special 

 hackles and curved tail-feathers usuallv seen in cocks. 

 This must be due to their possessing a secretion 

 similar to that of an ordinary hen, for when their 

 reproductive organs are removed, we find, paradoxical 

 as It may seem, that they assume normal male 

 plumage as the result ! In Insects, as indicated 

 already, removal of the reproductive organs has no 

 effect upon other sex characters. 



We next come to certain strange abnormalities 

 which throw considerable light upon our problem. 

 .\mong Insects, curious individuals are found from 

 time to time in which some part of the body — usually 

 a half or a quarter — is male in character, yvhile the 

 rest is female. These are known as gynandromorphs. 

 In ants, \ery remarkable appearances may result. 

 The male ant is winged, the worker female wingless ; 

 and thus a gynandromorph maybe winged on one side 

 only (see Fig. 2). In the fruit-fly, not only the sex and 

 the secondary .sexual characters, but also the sex- 

 linked characters may be different on the two sides. It 

 has recently been shown that the gvnandromorphs we 

 have mentioned are reallv female In constitution, but 

 that at one of the early divisions of the egg, one of 

 the X chromosomes lai-s behind and fails to sret incor- 



