1 66 BIOLOGICAL LECTURES. 



cytes of Bombyx) ; and Julin. Keuten finds in Ceratiii^n a 

 " Nucleolo-Centrosoma," which is supposed to unite the char- 

 acteristics of nucleolus and centrosome, but the nature of this 

 body is still problematical, and Lauterborn found a nucleolus 

 but no centrosome. 



The great mass of evidence goes to show that there is no 

 relation between nucleoli and centrosomes, and the conclusion 

 that such a connection exists is probably incorrect. With cer- 

 tain stains it is possible to color the nucleolus like the centro- 

 some, but there are other methods which in most cases serve 

 to differentiate the two. The centrosomes are frequently ap- 

 parent at an early stage of the prophase, while the nucleolus 

 is still present within the nucleus (as I have observed in Pisci- 

 cola and Euchistiis), so that in such cases no genetic relation of 

 the two is possible ; and it would be contrary to all well-founded 

 ideas to conclude that an inert mass like the nucleolus is capable 

 of becoming a dynamic centre of the cell. 



7. Nature and Function of the Nucleolus. — From a careful 

 and unbiased study of the subject, one is led to conclude that 

 the true nucleolus is an inert mass and not an organ. Its 

 substance appears to be extranuclear in origin, as already 

 discussed, and at the time of its first appearance would not 

 seem to be a derivative of the chromatin, as maintained by 

 Hacker. Since nucleolar substance is found in the srreatest 

 abundance in cells which are undergoing a rapid process of 

 growth, as is well shown in the growth period of ovarian ova, 

 its amount would seem to stand in a pretty direct relation to 

 the intensity of nutritive metabolism ; and this is the reason 

 why its substance may well be considered a product of nutri- 

 tive metabolism. Exactly what kind of a product, we are not 

 yet in position to decide ; it may represent accumulated nutri- 

 tive material, or waste products of such material, or both. 

 Perhaps it is at an earlier stage a mass of mixed nutritive mat- 

 ter, from which the other elements of the nucleus gradually 

 extract the nourishment, until finally only indigestible sub- 

 stances are left in it. Possibly chromatin debris may become 

 mingled with the nucleolar substance, but there is good reason 

 for supposing that the nucleolar substance is not wholly a 



