20 Tin: CITROMOSOMES 



between the separate elements. On the other hand, 

 this explanation eannot be put forward for the 

 ' attachment fibres ' connected with the chromo- 

 somes. These, liowever, have never been seen in 

 living material and are invisible in well-fixed material 

 in the majority of cases ^^' i^' ^^^ ; they are most 

 probably caused by shrinkage of the spindle in the 

 fixing solution in those cases where they can be 

 seen. ^2 xhe term ' spindle fibre ' should thus be 

 dropped, at any rate until much more definite evidence 

 is available ; whatever is the physical basis of the 

 artefact it is not a fibre ; nor is the conception that 

 ' spindle fibres ' arise from ' lines of force ' a useful 

 one. If, as I believe, these structures arise from 

 shrinkage of the spindle forming wrinkles or other 

 longitudinal distortion-artefacts on the surface of 

 the spindle elements it is only natural that these 

 should be associated with any solid object such as 

 the attachment point of a chromosome, which 

 interrupts the substance of the spindle element. 

 Lewis and Lewis ^^^ have shown that it is possible 

 to produce ' spindle fibres ' in living tissue-culture 

 cells by the use of acid media ; the phenomenon is 

 reversible, since on subsequently raising the ^jH the 

 structures disappear. 



We left the chromatids in prometaphase as a pair 

 of thickened threads lying closely approximated 

 tliroughout their length and only interrupted by 

 the spindle attachment and the secondary con- 

 strictions (if any). Usually they come to lie even 

 closer together at metaphase, so that the visible 

 * split ' between them disappears ; a metaphase 

 chromosome in transverse section thus has the shape 

 of the symbol oo. By most ordinary methods of 

 fixation the metaphase chromatids show no trace 

 of internal structure, but a2)pear as homogeneous 

 cylindrical rods (hi some cases they are slightly club- 

 shaped having a greater diameter at the distal ends 

 than at the spindle attachment). By various special 



