176 SECTIONAL ADDRESSKS. 



separate chromosomes at corresponding loci at each successive mitosis. 

 The reduction from the diploid to the haploid number, according to 

 the more generally accepted interpretation of the appearances during 

 the meiotic phase, is due to the adhering together in pairs of homologous 

 chromosomes, each member of the set originally received from one 

 parent lying alongside and in close contact with its mate received from 

 the other. Later these bivalent chromosomes are resolved into their 

 components so that the two groups destined one for either pole consist 

 of whole dissimilar chromosomes, which then proceed to divide again 

 longitudinally to furnish equivalent half chromosomes to each of the 

 daughter nuclei. According to the view of Farmer the homologous 

 chromosomes do not lie alongside, but become joined end to end. The 

 longitudinal split seen in the bivalent structure is interpreted as a 

 separation not of whole chromosomes but of half chromosomes already 

 formed in anticipation of the second division of the meiotic phase. As 

 however on either interpretation the same result is ultimately secured, 

 viz. : the distribution of whole paternal and maternal chromosomes to 

 different nuclei which now contain the haploid number, it is not essen- 

 tial to our present purpose to discuss the cytologioal evidence in support 

 of these opposing views in fuither detail. Nor, indeed, would it 

 be practicable within the limits of this Address. The obvious close 

 parallel between the behaviour of the cliromosomes — their pairing and 

 separation — and that of Mendelian allelomorphs which similarly show 

 pairing and segregation, first led to the suggestion that the factors 

 controlling somatic characters are located in these structures. The 

 ingenious extension of this view which has been elaborated by Morgan 

 and his co-workers presumes the arrangement of the factors in linear 

 series after tlie manner of the visible chromomeres— tlie bead-like 

 elements which can be seen in many organisms to compose the 

 chromatin structure — each factor and its opposite occupying correspond- 

 ing loci in homologous chromosomes. From this conception follows 

 the important corollary of the segregation of the factors during the 

 process of formation and subsequent resolution of the bivalent chromo- 

 somes formed at the reduction division. We should suppose, according 

 to Morgan, in the case of characters showing independent inheritance 

 and giving identical Mendelian ratios whichever way the mating is 

 made, and however the factorial combination is brought about, that the 

 factors controlling the several characters are located in different 

 chromosomes. Thus, in the case of Datura already mentioned, the two 

 factors affecting sap colour and prickliness respectively would be pre- 

 sumed to be located so far apart in the resting chromatin thread that 

 when separation into chromosomes takes place they become distributed 

 to different members. Where unrelated character's show a linhed 

 inheritance the factors concerned are held on the other hand to lie so 

 near together that they are always located in one and the same chromo- 

 some. Furthermore, and here we come to the most debatable of the 

 assumptions in Morgan's theory, when the bivalent chromosome com- 

 posed of a maternal and a paternal component gives rise at the reduction 

 division to two single dissimilar chromosomes, these new chromosomes 

 do not always represent the original intact maternal and paternal 



