284 



BIOLOGICAL LECTURES. 



any desired part, as is seen in the formation of the caryokinetic 

 spindle, for the separation of the chromosomes. The aster, from 



this point of view, may be 

 considered as a physiological 

 device for concentrating the 

 cytoplasmic substance in a 

 form which can be spun out 

 again into filaments in the 

 direction which will produce 

 a definite physiological effect. 

 (2) The second function of 

 the aster and the centrosome 

 is quite different. I refer to 

 the function of the aster in 

 the pigment cell and its like. 

 The centrosome in such cases 

 becomes the incidental prod- 

 uct, due to the fusion of the 

 proximal ends of the aster 

 fibrils. It is the fibril itself, 

 however, that is chiefly uti- 

 lized in such cells. By the 

 lengthening and shortening 

 of the fibril the shape and apparent size of the cell are changed, 

 which is the essential characteristic of a pigment cell. 



Thus, it appears probable that the two parts of the radial 

 system of the cell, by which I mean the compound structure 

 composed of the centrosome and its peripheral rays, have 

 different functions in different cells. In cells dividing caryo- 

 kinetically the centrosome is chiefly utilized, and in pigment- 

 cells and their like, the peripheral rays. In all cases the cen- 

 trosome and ray-like fibrils are the modification of the cyto- 

 plasm, but the uses to which the two respective parts are put 

 are quite different. The view that the centrosomal portion of 

 the radial system is chiefly utilized in the caryokinetic division 

 of the nucleus is further rendered probable by the existence 

 of free centrosomes at the poles of the spindle, without any 

 visible rays around them. 



Fig. 9. The spermatocyte of Salamander, 

 showing the formation of a small spindle (sp) be- 

 tween the two centrosomes, CC ; ch, chromosome. 

 (After Hermann.) 



