The Bearing of Cytological Research on Heredity. 335 



heredity was not yet sufficiently advanced for a full appreciation of its 

 significance ; but some very interesting theoretical suggestions had been 

 offered by Eoux, Weismann, de Vries, and other writers. While most of 

 these hardly admitted of actual verification, two nevertheless proved to be 

 of especial importance to later research. One was the pregnant suggestion 

 of Eoux (1883), that the formation of chromosomes from long threads brings 

 about an alignment in linear series of different materials or " qualities." By 

 longitudinal splitting of the threads all the " qualities " are equally divided, 

 or otherwise definitely distributed, between the daughter-nuclei. The other 

 was Weismann's far-seeing prediction of the reduction division, that is to say, 

 of a form of division involving the separation of undivided whole chromosomes 

 instead of the division -products of single chromosomes. This fruitful suggestion 

 (1887) pointed out a way that was destined to lead years afterwards to the 

 probable explanation of Mendel's law of heredity. 



(2) Such, in bird's-eye view, were the most essential conclusions of our 

 science down to the close of the nineteenth century. A new era of discovery 

 now opened. As soon as the Mendelian phenomena were made known it 

 became evident that in broad outline they form a counterpart to those 

 which cytology had already made known in respect to the chromosomes. 

 Characters and chromosomes alike are singly represented (haploid or simplex) 

 in the gametes, doubly represented (diploid or duplex) in the zygote and its 

 products. In the formation of new germ-cells both alike are once more 

 reduced from the diploid to the haploid condition. A parallelism so striking 

 inevitably suggested a direct connection between the two orders of 

 phenomena. Aud the hope was thus raised that the mechanism of heredity 

 might be susceptible of a far more searching analysis than had yet been 

 thought of. 



It is a rather striking coincidence that almost at the moment of the 

 re-discovery of Mendel's law, and apparently quite independently of it, 

 microscopical studies were establishing the cytological facts upon which its 

 explanation probably rests. Guyer's studies on hybrid pigeons led him, in 

 1900, to suspect a disjunction of maternal and paternal ehromatin-elements 

 in the reduction division, a conclusion which he developed further in 1902. 

 But the real basis for an explanation of Mendel's law was laid by two 

 conclusions announced in 1901 by Boveri and by Montgomery, independently 

 of each other and apparently without knowledge of the Mendelian phenomena. 

 Familiar as these conclusions are, I will dwell upon them for a moment, since 

 they are fundamental to all that follows. 



Boveri's masterly experiments on dispermic sea-urchin eggs gave the first 

 conclusive proof that the chromosomes directly affect the process of 



