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SCIENCE 



[N. S. Vol. XLIV. No. 1137 



been an interchange or " crossing-over " be- 

 tween the two homologous chromosomes. We 

 know that such new combinations can be 

 formed. Gametes bearing them are produced 

 in all cases in which the coupling or the 

 repulsion— to use the older terms — is not com- 

 plete. To account for the crossing-over of 

 factors from one chromosome to its mate Mor- 

 gan appeals to certain phenomena of twist- 

 ing and interlacing of chromosomes in synap- 

 sis, first made prominent by Janssens, who 

 observed them in Amphibia. It is suggested 

 that in the course of this process of twisting 

 the chromosomes may anastomose and again 

 break, exchanging parts of their substance. 

 For those unversed in practical cytology it is 

 not quite easy to judge how far this hypoth- 

 esis is in accord with observed fact. That 

 twisting takes place in many types, especially 

 Amphibia, is clear; but neither the figures 

 reproduced from Janssens nor the originals 

 from which they are taken — still less the very 

 fragmentary observations of both Stevens and 

 Metz from Drosophila — provide more than a 

 slender support for this most critical step in 

 the argument. It is to be hoped that the 

 authors will before long tell us exactly upon 

 what evidence they are here relying. 



The formation, then, inside a linked group, 

 of factorial combinations other than those 

 which entered from the parents, is ascribed 

 to crossing-over from one chromosome to its 

 fellow or mate. At an early stage in the 

 work, the curious and very significant fact 

 was observed that in the male no such cross- 

 ing over took place in regard to the various 

 factors which had been proved to be sex-linked. 

 The cytological interpretation of this discov- 

 ery was ready to hand. In many forms, espe- 

 cially insects, the sperms have been proved 

 to be of two kinds, those possessing an X 

 chromosome, destined to form females, and 

 those without this chromosome, destined to 

 form males. If therefore the X chromosome 

 carries the sex-linked factors — a supposition 

 inevitable inasmuch as these factors are all 

 destined to go into the daughters — and if 

 there is no real mate to the X chromosome, 

 evidently there can be no interchange or cross- 



ing-over here. Therefore in the case of sex- 

 limited characters linkage is complete. 



On tracing the growth of the theory or group 

 of theories which have been built up on the 

 Drosophila evidence the consideration just 

 propounded stands out as the original founda- 

 tion-stone. It was so introduced in the chief 

 inaugural paper of the series. This " sex 

 chromosome in the male has no mate," Morgan 

 tells us, and consequently no interchange with 

 it takes place. 1 



On reference, however, to the work of Miss 

 Stevens (1908) whose paper is given as author- 

 ity for the mateless condition of the X chromo- 

 some in Drosophila ampelophila, we read that 

 she found extreme difficulty in studying the 

 cytology of this creature, but ultimately satis- 

 fied herself that there is an unequal pair. The 

 more recent cytological work of Metz relates 

 entirely to the female, but in a note on the 

 male he remarks 



so far as my observations go, they indicate an 

 unequal XY pair in the male, without any addi- 

 tional piece attached to either. Neither my ob- 

 servations nor those of Miss Stevens are conclu- 

 sive, however, owing to the difficulty of observing 

 the chromosomes in these stages. The question is 

 important for the bearing it has upon the breed- 

 ing experiments with this fly, and we are doubly 

 unfortunate in being thus far unable to settle it. 2 



In 1913, Sturtevant in introducing the first 

 formal development of the theory of linear 

 arrangement, presently to be considered, re- 

 peats that there is no crossing over among the 

 sex-linked group of factors in the male, 

 " since the male has only one sex-chromo- 

 some." 3 When we come to the book of 1915 

 the same authors have an entirely different 

 conception of the cytological phenomena. 

 There are two sex chromosomes in the male, 

 and though as a matter of convention, one of 

 them is represented as different from the other 

 in shape, the reader is very properly told that 

 the distinction has not yet been observed. 4 



i<7. Exp. Zool., 1911, XI., p. 383. 



2 J. Exp. Zool., 1914, XVII., p. 49, note. 



a Sturtevant, J. Exp. Zool, 1913, p. 44. 



* In the recent paper of Bridges (Genetics, I., 

 1916) the distinction in shape is stated to be a 

 reality. 



