1905.] Occurrence of Heterotypic al Mitoses in Cancer. 



229 



As a consequence the longer limb does not come to a position of equilibrium 

 in the equatorial plane, but may take up one inclined to the axis of the 

 achromatic figure. When the attraction fibres are attached to the chromo- 

 somes in this manner in a cell of elongated form, the longer limbs may even 

 come to lie parallel to the spindle axis, and nuclear divisions closely 

 resembling heterotypical mitoses may result. In such mitoses pairs of 

 distinct chromosomes whose longer limbs lie on opposite sides of the 

 equator, while their apices ai*e closely opposed, simulate bivalent chromo- 

 somes, cf. figs. 5 to 10. In polar view the apparent halving of the 

 chromosome number due to superposition, and the unusual vertical extension 

 of the free longer limbs, are even more deceptive ; unless the longitudinal 

 splitting of the chromosomes is very clear, the resemblance to heterotypical 

 mitoses may be almost perfect. During the separation of the daughter 

 chromosomes such nuclear divisions are especially deceptive, because the 

 longer limbs adhere for some time after separation of the apices and short 

 limbs. Barrel-shaped forms result, in which the crowding together of the 

 chromosomes renders their enumeration impossible, but at the same time 

 conveys the impression of a diminution in their number. 



A much more serious source of error results from individual .differences 

 in the size of the chromosomes in one and the same nucleus. Montgomery* 

 and Suttont have drawn attention to this phenomenon in the sexual cells of 

 invertebrates. We have found it to be present also in many vertebrate 

 mitoses. When the chromosomes attain the position of equilibrium in the 

 equatorial plate the smaller take up a position nearer the axis of the central 

 spindle than the larger or more massive ones. Seen in profile the larger 

 may then completely screen the smaller from view, and lead to an under- 

 estimate of. the chromosome number. 



Normally, nuclear division takes place by means of bipolar mitosis distri- 

 buting the halves of the chromosomes to two daughter nuclei. An interesting- 

 abnormality results, when, from any cause, this segregation of the daughter 

 elements does not take place. This may either result from the centrosome, 

 remaining single or, if after division of the centrosome, one only become 

 attached to the chromosomes by the attraction fibres. The chromosomes 

 then remain in one group, but the daughter elements separate slightly from 

 each other before combining to form one large nucleus, containing twice as 

 many chromosomes as the mother nucleus. This form of mitosis, known as 

 a " monaster " (from the presence of only one active attraction sphere) has been 



* T. H. Montgomery, ' Proc. Acad. Nat. Sc., Philadelphia,' 1901 ; 'Biol. Bull.,' vol. 4, 

 1903. 



t W. S. Sutton, ' Biol. Bull.,' vol. 4, 1902. 



VOL. LXXVII. — B. R 



