366 



SCIENCE. 



[N. S. Vol. XII. No. 297. 



karyokinetic figures corresponding numbers 

 are found in homologous cells, seems to be 

 not improbable. 



In the preceding description I have in- 

 cidentally referred to the appearance in the 

 pi'oliferating cell of an archromatic spindle- 

 like figure. Although this was recognized 

 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 surrounds the 

 nucleus. In the center of each pole is a 

 somewhat opaque spot (central body) sur- 

 rounded by a clear space, which, along with 

 the spot, constitutes the- centrosome or the 

 sphere of attraction. From each centro- 

 some extremely delicate lines may be seen 

 to radiate in two directions. One set ex- 

 tends 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 radia- 

 tions 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 achro- 

 matic figure, or the entire figure, exists in 

 the cell as a permanent structure in its rest- 

 ing phase ; or if it is onlj^ 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 centi'osome in 

 the pole of the spindle, is a readily recog- 

 nizable object in each cell. The polar and 

 spindle-like radiations are in evidence dur- 

 ing 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 di- 

 vision 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 -nnth its sphere in leucocytes, as well 

 as in epithelial cells and those of other tis- 

 sues. Subsequently Heidenhain and other 

 histologists have recorded similar observa- 

 tions. 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 uni- 

 versally entertained. If not always capable 

 of demonsti'ation in the resting stage of a 

 cell, it is doubtless to be regarded as po- 

 tentially present, and ready to assume, 

 along with the radiations, a characteristic 

 appearance when the process of nuclear di- 

 vision is about to begin. 



One can scarcely regard the presence of 

 so remarkable an appearance as the achro- 

 matic 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 conti'actile in their properties. Others, 

 again, look upon them as morphological ex- 

 pressions of chemical and dynamical energy 

 in the protoplasm of the cell body. On 

 either theory we may assume that they in- 

 dicate an influence, emanating, it may be, 

 from the centrosome, and capable of being 

 exercised both on the cell plasm and on the 

 nucleus contained in it. On the contractile 

 theory, the radiations which form the body 

 of the spindle, either by actual traction of 

 the supposed fibrillse or by their pressure 

 on the nucleus which they surround, might 

 impel during karyokinesis the dividing 

 chromosome elements towards the poles of 



