The Bearing of Cytological Research on Heredity. 341 



mediate conditions, varying with the species, and X may consist of two or 

 more separate chromosomes. The Y-chromosome, on the other hand, is 

 single in all cases thus i'ar accurately known.* 



At the time these conclusions were announced it seemed unlikely that the 

 fertilisation of the egg by the two classes of spermatozoa could ever actually 

 be followed out. This has, nevertheless, been accomplished recently by 

 Mulsow in the case of a nematode, Ancyrocanthus, where the chromosomes 

 remain distinct in the mature spermatozoa and can readily be counted, even 

 in the living object. Both classes were here traced into the egg, and the 

 sexual differences were clearly shown in the germ-nuclei at the time of their 

 union. 



I will not enter upon the many interesting modifications of detail which 

 these phenomena exhibit. In principle, the facts are the same in many 

 insects and nematodes, probably in the myriapods and arachnids, perhaps 

 also in the mammals and in man, though the demonstration here still leaves 

 much to be desired. An extremely interesting series of researches by 

 Morgan, von Baehr, Schleip, Doncaster, and others have proved that the 

 same principle applies also to the parthenogenetic forms, such as the aphids, 

 bees, and ants. Hardly less interesting are the investigations, especially of 

 Boveri, Schleip, Kriiger, and Zarmk, which show that this principle may 

 even be extended to certain types of hermaphrodites. The results of genetic 

 experiments on Lepidoptera and on birds lead us to expect the existence in 

 these forms of a different cytological type, in which the eggs, instead of the- 

 spermatozoa, are of two different classes ; but the cytological facts have not 

 yet become sufficiently clear to warrant any definite conclusion. In the case 

 of birds, indeed, a conspicuous contradiction still appears between the 

 cytological and the genetic results ; but the cytological observations have not- 

 yet produced evidence that can compare in cogency with that available in 

 case of the insects or the nematodes. 



I turn to the broader significance of the cytological facts that have been 

 made known in this field. They constitute a very definite advance upon 

 Boveri's general demonstration of the qualitative differences of the chromo- 

 somes ; for it is impossible to doubt that the X-chromosome stands in some 

 special causal relation with sex-heredity. A powerful argument for this is- 

 given by the facts of sex-linked heredity, which I shall presently consider. 

 The riddle which this form of linkage presents is solved by a cytological 

 phenomenon to which I first drew attention in 1906. The Y-chromosome,. 



* In an extreme case, now under investigation by Mr. Goodrich in my laboratory, the 

 X-element consists of not less than eight distinct chromosomes, opposed by a single Y. 

 The females here show seven more chromosomes than the males (photographs). 



