THE GENERAL OUTLINE OF MITOSIS 19 



Darlington ^^ regards this as the actual organ of 

 attachment and calls it the ' attachment chromo- 

 mere ' or ' centromere '. I have, however, ^^^ given 

 reasons for believing that it is the non- staining 

 region which is the true attachment -organ ; in 

 many cases the ' centromere ' cannot be seen in the 

 middle of the non-staining region, although it may 

 be below the limit of visibility in these cases. 



Apparently the spindle attachment, unlike the 

 rest of the chromosome, remains undivided during 

 prophase and only divides at prometaphase ; its 

 two halves then organize a spindle element, above 

 and below the equatorial plane. Up till now we 

 have only been considering ordinary * bipolar ' 

 spindles ; but bipolarity is not an essential feature 

 of the spindle — a fact which eliminates theories of 

 mitosis based on a superficial analogy with electrical 

 or magnetic models. In many cells such as those of 

 cancerous tissues and in Sea -Urchin eggs which have 

 been fertilized several times as a result of poly- 

 spermy, multipolar spindles with a number of 

 equatorial planes intersecting one another are 

 found 1, 10, 143 . there may be as many as 12 poles 

 and 6 equatorial planes. The probable structure 

 of these multipolar spindles is indicated in Fig. 36. 



Even more interesting than the multipolar spindles 

 are the'unipolar ones (half -spindles) found at meiosis 

 in some insects (Fig. 3c) and described by the 

 ScHRADERS '^^' ^^^ and by Metz, Moses and Hoppe.^^s 

 At present no useful suggestion can be put forward 

 as to how they are formed. 



So far we have said nothing of the ' spindle fibres ' 

 described in many text-books, but have considered 

 the spindle as a bundle of ' elements ' corresponding 

 in number with the chromosomes, with or without 

 the addition of a central element between the 

 centrosomes. There appears to be little doubt that 

 many of the ' continuous ' or ' interzonal ' fibres 

 described by various workers were in fact fissures 



