PROTOPLASM AND THE CELL. 7 



In addition to these there is, however, still another body to 

 be mentioned which is especially evident in cells which are 

 undergoing multiplication, but which has also been found in 

 various resting cells, especially in lymph-corpuscles, various 

 kinds of endothelial cells, and in pigment-cells. This is the 

 structure known as the centrosome (Fig. 1, c). It is usually 

 an exceedingly small spherical body which does not readily 

 stain with the reagents which place the chromatiu in evi- 

 deuce, but has a strong affinity for certain acid aniline stains, 

 such as safraniu, fuchsiu, or orange. Usually but a single 

 centrosome is present in each cell, though occasionally two or 

 even more may occur, and it is situated in the cytoplasm in 

 the neighborhood of the nucleus, sometimes resting in a 

 slight concavity on the surface of that structure. Surround- 

 ing the centrosome there is frequently to be seen, more 

 especially in dividing cells, a radial arrangement of the cyto- 

 plasmic reticulum, the centrosome being comparable to a 

 star from which rays pass out in all directions, whence the 

 term aster which is applied to the combination of the cen- 

 trosome and the cytoplasniic rays. 



The significance of the centrosome will be seen later when the phe- 

 nomena of cell-division are under consideration, but its origin may be 

 inquired into at this place. Two views are current in regard to this 

 matter, according to one of which the centrosome has its origin in the 

 nucleus and at a certain period of the cell's existence is extruded from it. 

 In favor of this view the intimate association of the centrosome and the 

 nucleus are pointed out, an association which becomes especially pro- 

 nounced during cell-division, the astral rays connected with the cen- 

 trosome appearing to penetrate the nucleus and in fact to bring about its 

 division into two parts. According to the other theory, however, the cen- 

 trosome is a constituent of the cytoplasm and in its origin has nothing to 

 do with the nucleus. Quite recently an interesting amplification of this 

 idea has been suggested to the effect that the centrosome is nothing more 

 or less than an aggregation of cytoplasm ic microsomes. The astral rays 

 are cytoplasmic fibres converging from all sides, and since microsomes 

 occur along their course an aggregation of these bodies might be found 

 where the fibres meet. This idea cannot be discussed in detail here, but 

 it may be pointed out that the absence of a centrosome in cells which have 

 lost their powers of reproduction is readily explicable on this idea, the ag- 

 gregated microsomes having scattered in such cells, and furthermore 

 that the theory explains certain peculiar arrangements replacing the 

 typical aster during the division of some cells. 



