184 . - INTRODUCTION TO CYTOLOGY 



Sakamura (1920), although holding the fibers to be important agents 

 in the normal separation of the daughter chromosomes, observes that in 

 abnormal nuclear divisions where no fibers are present the chromosomes 

 still show movements which are probably due to streaming of the cyto- 

 plasm and to surface tension phenomena. 



The relation of streaming and surface tension to cytokinesis will be 

 discussed in the section dealing more particularly with cytokinesis. 



Osmosis. In a theory of the mechanics of karyokinesis proposed by 

 Lawson (1911) the principal factor involved is osmosis. Lawson's ex- 

 planation is essentially as follows. During the late prophase karyolymph 

 passes outward through the nuclear membrane by osmosis, this loss of 

 fluid resulting in a contraction of the nucleus. Owing to the fact that 

 the cytoplasmic reticulum is continuous with the nuclear membrane this 

 contraction sets up radial lines of tension in this reticulum on all sides of 

 the nucleus. As the process continues these lines or "fibers" gradually 

 become arranged in two opposed groups, while the nuclear membrane to 

 which they are attached continues to contract until it actually enwraps 

 each double chromosome. To each double chromosome there are thus 

 attached fibers which represent stretched and distorted regions of the 

 cytoplasmic reticulum extending to the two sides of the cell. When the 

 chromosomes become properly arranged at the equatorial plane the 

 fibers, which are under considerable tension, are able to pull the daughter 

 chromosomes apart and draw them to the poles. As the fibers relax they 

 resume their true reticular state. Although the chromosomes are thus 

 drawn apart by the shortening of "fibers" attached to them, Lawson 

 points out that this is not to be regarded as a case of true active contrac- 

 tility, but only as a release of tension set up in the passive but elastic 

 cytoplasmic reticulum as the result of the exosmosis of karyolymph from 

 the nucleus. This theory has been severely criticized by a number of 

 writers, chiefly on the grounds that such an enwrapping of the chromo- 

 somes by the nuclear membrane as Lawson describes cannot be demon- 

 strated in many objects subsequently examined, and that the membrane 

 frequently goes into solution when both it and the growing fibers are 

 still some distance from the chromosomes. 



Electrical Theories. The striking resemblance between the achromatic 

 figure and the lines of force in an electromagnetic field e.arly led to at- 

 tempts to account for mitosis on the basis of electrical principles. Several 

 investigators, working with various chemical substances, succeeded in 

 modelling fields of force that illustrated graphically the changes supposed 

 to take place in the dividing cell. In later years the electromagnetic 

 interpretation was again brought into prominence by Gallardo, Hartog, 

 and Prenant. At first Gallardo (1896) believed the two spindle poles 

 to be of unlike sign, but later (1906), as the result of the researches of 

 Lillie (1903) (see p. 62) on the behavior of nucleus and cytoplasm in 



