196 \V. E. AGAR 



characteristic and constant for any given chromosome. This 

 shape is commonly some form of V, with equal or unequal 

 limbs. The point at which the chromosome bends to form 

 the V — whether in the middle or towards one end — (which 

 is also the spot to which the spindle-fibre is attached) varies 

 from chromosome to chromosome, but is constant for any 

 given chromosome. Similarly, the transverse constrictions 

 which develop across the chromosomes of so many organisms 

 are constant in position for a given chromosome. This con- 

 stancy in position is proof of the existence of a constant 

 differentiation, in a lengthwise direction, of the substance of 

 the chromosome. 



The tendency of certain chromosomes to develop transverse 

 constrictions at spots constant for each particular chromosome 

 is specially significant in estimating the value of the criticism 

 that chromomeres are 'artefacts'. In Lepidosiren 

 (Agar, 1913) the long somatic chromosomes usually show no 

 trace of a transverse constriction. Each chromosome is a 

 smooth curved rod or V of approximately uniform thickness. 

 When the chromosomes become shorter and thicker, as happens 

 regularly in the meiotic phase, and occasionally, from unknown 

 reasons, in somatic tissues also, the transverse constrictions 

 develop in a spot characteristic for each particular chromosome 

 (which is also the point at which the apex of the V is situated, 

 when the chromosome is in this shape). 



This shortening and thickening of the chromosomes was 

 produced in several plants by Sakamura (1920) by the action of 

 various reagents, such as chloral hydrate and chloroform. 

 These artificially shortened chromosomes showed well-marked 

 and characteristically placed constrictions, though the position 

 of these is barely indicated in normally fixed tissue. The con- 

 stancy in position of these constrictions shows that they can 

 only be called ' artefacts ' in the sense that they make visible 

 a pre-existing heterogeneity of the chromosome substance, 

 which is concealed from view in the ' typical ', well-fixed, and 

 apparently uniform chromosome. Indeed, to deny that the 

 chromomeres correspond to pre-existing local differentiations 



