ADDRESS BEFORE THE BRITISH ASSOCIATION. 573 



The pole at each end of the spindle lies in the cell plasm which 

 surrounds the nucleus. In the center of each pole is a somewhat 

 opaque spot (central hody) surrounded by a clear space, which, along 

 with the spot, constitutes the centrosome of the sphere of attraction. 

 From each centrosome extremely delicate lines may he seen to radiate 

 in two directions. One set extends towards the pole at the opposite 

 end of the spindle, and, meeting or coming into close proximity with 

 radiations from it, constitutes the body of the spindle, which, like a per- 

 forated mantle, forms an imperfect envelope around the nucleus during 

 the process of division. The other set of radiations is called the polar 

 and extends in the region of the pole towards the periphery of the cell. 



The question has been much discussed whether any constituent part 

 of the achromatic figure, or the entire figure, exists in the cell as a 

 permanent structure in its resting phase; or if it is only present during 

 the process of karyokinesis. During the development of the egg the 

 formation of young cells, by division of the segmentation nucleus, is so 

 rapid and continuous that the achromatic figure, with the centrosome in 

 the pole of the spindle, is a readily recognizable object in each celL, The 

 polar and spindle-like radiations are in evidence during karyokinesis, 

 and have apparently a temporary endurance and function. On the other 

 hand, van Beneden and Boveri were of opinion that the central body of 

 the centrosome did not disappear when the division of the nucleus came 

 to an end, but that it remained as a constituent part of a cell lying in the 

 cell plasm, near to the nucleus. Flemming has seen the central body 

 with its sphere in leucocytes, as well as in epithelial cells and those of 

 other tissues. Subsequently Heidenhain and other histologists have re- 

 corded similar observations. It would seem, therefore, as if there were 

 reason to regard the centrosome, like the nucleus, as a permanent con- 

 stituent of a cell. This view, however, is not universally entertained. 

 If not always capable of demonstration in the resting stage of a cell, it 

 is doubtless to be regarded as potentially present, and ready to assume, 

 along with the radiations, a characteristic appearance when the process 

 of nuclear division is about to begin. 



One can scarcely regard the presence of so remarkable an appear- 

 ance as the achromatic figure without associating with it an important 

 function in the economy of the cell. As from the centrosome at the 

 pole of the spindle both sets of radiations diverge, it is not unlikely 

 that it acts as a center or sphere of energy and attraction. By some 

 observers the radiations are regarded as substantive fibrillar structures, 

 elastic or even contractile in their properties. Others, again, look upon 

 them as morphological expressions of chemical and dynamical energy 

 in the protoplasm of the cell body. On either theory we may assume 

 that they indicate an influence, emanating, it may be, from the centro- 

 some and capable of being exercised both on the cell plasm and on the 



