14 REPORT — 1900, 



In the study of karyokinesis, importance has been attached to the 

 number of chromosomes in the nucleus of the cell. Flemming had seen 

 in the Salamander twenty-four chromosome fibres, which seems to be a 

 constant number in the cells of epithelium and connective tissues. In 

 other cells again, especially in the ova of certain animals, the number is 

 smaller, and fourteen, twelve, four, and even two only have been described. 

 The theory formulated by Boveri that the number of chromosomes is con- 

 stant for each species, and that in the karyokiuetic figures corresponding 

 numbers are found in homologous cells, seems to be not improbable. 



In the preceding description I have incidentally referred to the appear- 

 ance in the proliferating cell of an achromatic spindle-like figure. Al though 

 this was recognised by Fol in 1873, it is only during the last ten or twelve 

 years that attention has been paid to its more minute arrangements and 

 possible signification in cell-division. 



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

 rounds the nucleus. In the centre of each pole is a somewhat opaque 

 spot (central body) surrounded by a clear space, which, along with the 

 spot, constitutes the centrosome or the sphere of attraction. From each 

 centrosome extremely delicate lines may be seen to radiate in two direc- 

 tions. One set extends towards the pole at the opposite end of the spindle 

 and, meeting or coming into close proximity with radiations from it, con- 

 stitutes the body of the spindle, which, like a perforated 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 recognisable 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 recorded similar observations. It would seem, therefore, as if there 



were reason to regard the centrosome, like the nucleus, as a permanent 



constituent 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^ 



