182 INTRODUCTION TO CYTOLOGY 



The nucleolus has been thought by some observers to furnish material 

 for the formation of the spindle, because of the fact that it very commonly 

 disappears from view at about the time the spindle begins to differentiate. 

 It is possible that in some cases there may be a connection of this sort 

 between nucleolus and spindle, but it is clear that this cannot serve as a 

 general interpretation of spindle origin. 



That the achromatic figure may arise from a special substance not 

 constantly present in the cell, but formed anew at each mitosis, is a 

 theory which several workers have advanced. The researches of Devise 

 (1914) and Miss Nothnagel (1916) may be cited for illustration. Devise, 

 as the result of a careful study of the development of the spindle in the 

 microsporocytes of Larix, concluded that the spindle is not formed by the 

 rearrangement of any preexistent nuclear or cytoplasmic structures, but 

 arises from a substance which develops in the nuclear region during the 

 late prophases (after diakinesis). He was not able to decide whether this 

 substance is of purely nuclear origin or is formed when the karyolymph 

 comes in contact with the cytoplasm. The interaction of karyolymph and 

 cytoplasm is emphasized by Miss Nothnagel in her work on Allium. 

 She points out that the contact of newly formed karyolymph with the 

 cytoplasm at telophase brings about the precipitation of the nuclear 

 membrane, and that in an analogous manner an exosmosis of karyo- 

 lymph through the nuclear membrane into the cytoplasm during prophase 

 causes the precipitation of fine fibrils around the nucleus, these fibrils then 

 developing into the spindle. The achromatic figure therefore arises 

 from a special substance, but this substance, as in the case of Larix, 

 is newly formed at each mitosis. 



Conclusion. — In general it may be said that although the spindle 

 fibers and the motor and contractile elements of the cell appear to have 

 a substantial relationship with one another, the substance common to 

 them is probably "not to be regarded as being necessarily a permanent 

 constituent of the cell, but only as a phase, more or less persistent, in 

 the general metabolic transformation of the cell substance" (Wilson). 

 Indeed the conspicuous tendency on the part of cytologists at present 

 is to regard the achromatic figure neither as a mere rearrangement of a 

 structure previously present, nor as a form assumed by a special spindle 

 substance, but rather as the result of streaming, gelation, and other 

 temporary alterations in the colloidal substratum. This interpretation 

 is strongly supported by the microdissection studies to be cited in a 

 subsequent paragraph. 



The Mechanism of Mitosis.— Since the phenomenon of mitosis was 

 first described there have been put forward a number of theories to ac- 

 count for the operation of the achromatic structures in bringing about the 

 separation of the daughter chromosomes and for the division of the cell. 

 Many of the suggestions undoubtedly contain elements of truth, but it 



