470 TRANSACTIONS OP SECTION D. 



groups of characters is a question difficult to decide. The experiments of 

 Wilson and his school on Drosophila and other insects suggest that they are. 

 The best instance is the so-called sex-chromosome, which is supposed to carry the 

 determiner of sex and of the qualities which are sex-limited. In some cases 

 the female nucleus possesses one more chromosome than the male, and there are 

 two kinds of spermatozoon, one with one more chromosome than the other. 

 Hence it is assumed that sex is fixed by the spermatozoon. But when two 

 species are crossed, differing in a secondary sexual character, the distribution 

 of this character in the hybrid and in the F, generation shows that it cannot 

 possibly be carried by the sex-chromosome. Moreover, in other cases 

 (Abraxas) the inheritance of characters in a cross between two varieties indicates 

 that there are two kinds of egg and one kind of spermatozoon. Yet no con- 

 stant chromosomal difference between the two kinds can be detected (Don- 

 caster). In other words, the odd chromosome may not be the cause of sex- 

 difference, but in itself the result of that sex-difference. 



The phenomena of meiosis, however, and their agreement in form with the 

 sort of segregation of qualities postulated by the Mendelian hypothesis, suggest 

 very strongly that determiners of various characters are situated in definite 

 pairs of chromatin units which become separated from one another at the 

 nieiotic division. Since, however, the number of allelomorphic characters can 

 in many cases be proved to be very much larger than the number of chromo- 

 somes, the individual chromosomes cannot represent these determiners. May 

 not the chromosomes be simply groups of these determiners adhering by mutual 

 chemical affinity under the peculiar chemical conditions obtaining in the cell in 

 the period preceding karyokinesis ? If this be the case, the apparent total dis- 

 appearance of chromosomes during the resting period could be accounted for. 



2. 'Die Discession of Ihe Chromosomes and Mitok'uietism. 

 By Professor Marcus IIartog, D.Sc. 



In previous papers the author has shown that the phenomena of the cell- 

 spindle may be interpreted as the expression of a field of dual force, centering 

 on the centrosomes as opposite poles, on the assumption that the achromatic 

 fibres are more permeable to the force than their surroundings, and lie along the 

 lines of force whose distribution they, of course, modify. The chromosomes are 

 also more permeable, and may be termed ^flexible indurAors.' All objections 

 that have been urged to this view have been shown to be based on neglect either 

 of accepted physical truths or of the realities of the field. But he was so far 

 unable to produce any adequate explanation of the discession of the chromosomes. 

 This deficiency was due to the omission to take note of the different behaviours 

 of isolated poles and of inductors. The former move along the lines of force ; 

 but not the latter, and their path in a uniform field had not yet been worked 

 out by the physicist save in very few, simple, cases. With the collaboration of 

 Mr. Philip E. Belas this problem has received an experimental solution, which 

 is demonstrated at this meeting before Section A. Undoubtedly the path of an 

 inductor in a uniform, field does not correspond with that of a chromosome in 

 the cell ; but the cell-field is not uniform, for it is traversed by the spindle- 

 fibres, stretching from pole to pole along the lines of force, and modifying their 

 distribution, as above stated. If we represent these in a magnetic field by 

 pieces of soft iron extending from pole to pole, we see at once that lines of 

 force converge to them on either side, their density increasing as we approach 

 the poles. Now, an inductor tends to move in the direction in which it will 

 include the maximum number of lines of force ; and in our models we find indeed 

 that a floating inductor will move along the iron representative of the spindle- 

 fibre. Thus the path of the chromosome is now seen to be in accordance with 

 theory. 



The separation of the sister-chromosomes at the equator is difficult to under- 

 stand if the field be uniform, for the force tending to separate two adjacent 

 inductors lying across the equator is indefinitely small ; again here the model 

 shows us how the force is modified by the presence of the achroraatin fibre. For 

 the lines of force from either pole are continuous through the inductors in the 

 uniform field ; but with the representative of the inductor stretching from pole 



