i 4 THE STRUCTURE OF CELLS 



masses, usually of a spherical or oval shape, known as Nucleoli. 

 These bodies long ago attracted the attention of investigators, and 

 it will be remembered that they were raised to a rank of con- 

 siderable importance by both Schwann and Remak. They usually 

 are easily stained, and thus were included amongst the chromatin 

 bodies of the nucleus, but subsequent investigation has shown that 

 they are, in many cases, widely different from nuclein. Two kinds 

 of nucleoli were distinguished by Flemming under the names of 

 eu- and pseudo-nucleoli respectively, the latter representing, at least 

 chiefly, aggregations of a substance which closely approximates to, 

 if it is not actually identical with, true nuclein. And more recent 

 investigations have tended to confirm the supposition advocated by 

 Zacharias, that the ordinary eu-nucleoli, so far from consisting of a 

 single substance such as pyrenin (Schwartz), are complex mixtures, 

 or else, at any rate, bodies which readily yield, by suitable treat- 

 ment, different substances of complex molecular composition. It is 

 true that the author just referred to arrived at the conclusion that 

 the nucleoli were destitute of phosphorus, but this view can hardly 

 be maintained, at least in all cases. 



Investigations on the nuclei of Protozoa and of some of the 

 lower plants seem to have shown that these nucleoli consist of 

 at leas^ two groups of substances, the one consisting of, or 

 approximating to, nuclein, the other more nearly resembling the 

 linin, or even the cytoplasm, in its staining and other reactions. 

 At any rate, the chromatin, which forms so obvious a character in 

 dividing nuclei, appears in some cases, e.g. Actinosphaerium, to be 

 mainly derived from a nucleolar source. It is highly probable that 

 these bodies are really heterogeneous, and represent reserves of 

 complex materials which can be drawn on for various purposes 

 during periods of nuclear activity. For at such times the nucleolus 

 always undergoes considerable change, and is either completely used 

 up, or its remains fragment and pass out into the cytoplasm, where 

 their further fate is still obscure. 



Whilst the nucleoli are thus losing substance they often exhibit 

 vacuolation, and even in resting nuclei vacuoles may sometimes be 

 detected within the nucleoli, pointing strongly to the correctness of 

 the hypothesis as to their heterogeneous nature. 



A further point which deserves mention in connection with the 

 nucleoli is the view held by some writers (e.g. R. Hertvvig) that 

 they stand in some close relation to the centrosomes, and that the 

 existence of the latter structures may be traced back to a nucleolar 

 origin. Further, Strasburger, in his account of kinoplasm, has 

 suggested that the nucleolar substance may serve as the material 

 which stirs up the dynamical and metabolic activities latent in the 

 cell. On the whole, it is impossible, in the present state of our 

 knowledge, to ascribe any single function to these bodies, and the 



