P L A S M 



than the closely related chromatin. It has a special 

 aflinity for acid aniline colors, gosin, etc. Its substance 

 has, therefore, been distinguished as plastin or para- 

 nnclcin. The nucleolus is especially found in the tissue- 

 cells of the higher animals and plants as an independent 

 constituent; it is wanting in many of the unicellular 

 protists. The same may be said of the centrosoma, or 

 "central body" of the cell. This is an extremely small 

 granule, on the very limit of visibility, the chemical com- 

 position of which is not known very well. We should 

 have paid no attention to this constituent of the cell 

 (distinguished in 1876) if it did not play an important, 

 and perhaps leading, part in indirect cell-division. As 

 the "polar body in the division of the nucleus," the 

 centrosoma exercises a peculiar attraction on the 

 granules distributed in the cytoplasm, which arrange 

 themselves radially about this centre. The centrosomata 

 grow independently and increase by cleavage, like the 

 chromoplasts (chlorophyll particles, etc.). When they 

 have split up, each of the daughter-microsomata acts in 

 turn as a centre of attraction on its half of the cell. 

 However, the great importance which modern cytologists 

 have ascribed to it on this account is discounted by 

 two circumstances. In the first place, we have not 

 succeeded, in spite of all efforts, in discovering a centro- 

 soma in the cells of the higher plants and many of the 

 protists; and, in the second place, a number of recent 

 chemical experiments have succeeded in producing 

 centrosomata artificially (for instance, by the addition of 

 magnesium chloride) in the cytoplasm. Hence many 

 cytologists regard the centrosoma as a secondary product 

 of differentiation in the cell-body, not the nucleus. 



Two other parts of the nucleus that we find very often, 

 but by no means universally, in the cells of the animal 

 and plant body are the nuclear membrane (caryotheca) 

 and the nuclear sap (caryolymph). A large number of 



141 



