Albert C. Eycleshymer 303 



are found, almost without exception, on the outer side of the cell. In 

 the adult fibres the nuclei are no longer found exclusively at the peri- 

 phery, but are scattered throughout the sarcoplasm and show no definite 

 arrangement either with reference to the planes of the animal or to the 

 axis or periphery of the cell itself. Why these nuclei come to lie within 

 the muscle cell is unknown. It is possible that with the continued 

 growth of the fibre their sphere of activity becomes too far removed from 

 that of cytoplasmic activity, necessitating a redistribution of nuclear 

 material. 



The nucleus thus undergoes a series of striking changes in position 

 which correspond to, if they are not correlated with, the shifting areas of 

 cytoplasmic activity. It is possible that the movement of the nucleus 

 from the axis of the cell to the periphery is the result of mechanical 

 factors, in that the continued formation of fibrillas from the inner toward 

 the outer side of the cell would cause a corresponding displacement of 

 the nucleus. To accoimt for their later position among the fibrillge 

 through the influence of mechanical factors is exceedingly difficult, but 

 becomes intelligible if we regard the movements as of physiological 

 significance. 



There are nuany observations which seem to show conclusively a physio- 

 logical correlation between nuclear movements and cyto])lasmic activity. 

 The most familiar instances are found in the various gland cells to which 

 nearly all histologists have called attention. In other animal cells the 

 same phenomena have been observed. I need but cite here the work of 

 Korschelt, 89, who found that in the ovarian eggs of a large number of 

 insects the nucleus moves toward the locality at which the egg receives 

 its nutriment and which must be interpreted as the area in which the 

 cytoplasmic activity is greatest. Some striking instances of like move- 

 ments are to be found among plants. Haberlandt, 87, from an extended 

 study of nuclear movements, concludes that the nucleus moves to the 

 area of greatest cytoplasmic activity, where it remains until the period 

 of local activity ceases, when it returns to its original position. A beau- 

 tiful illustration is found in the growth of epidermal hairs. In the root 

 hairs, where the growth is terminal, the nuclei are found at the ends of 

 the cells, but in the aerial hairs, where the growth takes place at the 

 base, the nuclei are found at the base of the cells. Tangl, 84, observed 

 that in the scales of Allium sepa the nuclei alw^ays gather at the points 

 where the cells have been injured. The same movements were observed in 

 Vaucheria by Haberlandt, showing that in regeneration the nuclei like- 

 wise migrate to the areas of accelerated cytoplasmic activity. 



