THE STRUCTURE OF CELLS 45 



mechanism is already present in the egg, and only an appropriate 

 stimulus (not the importation of a missing half of the machinery) 

 is required to set it in motion. Long ago Boveri suggested that 

 the centrosome rather than the nucleus was the important body 

 the introduction of which starts the process of segmentation, and 

 it may well be that his suggestion, in a modified form perhaps, 

 and without postulating an organisation of the specific excitatory 

 substance in so definite a form as a centrosome, may contain a con- 

 siderable element of truth. 



Amongst the lower organisms, as Klebs and others have shown, 

 the conditions favourable to the formation of sexual cells can be 

 largely referred to nutritive sources, and this is only another way 

 of saying that a definite stimulus ultimately working on the living 

 substance of the organism itself is responsible for the sexual reaction. 



But such a view of the matter leaves untouched the question 

 of the secondary utility of sexuality as a means of ensuring variation. 

 Indeed, this latter is perhaps best kept distinct from the primary 

 causes and conditions which first made sex not only a possible 

 but an inevitable incident in the life cycle of the greater part of 

 the higher organisms. 



Less obscure than its relations to the phenomenon of sex are 

 those which exist between the nucleus and the life of the cell. 



Gruber, Nussbaum, Verworn, and others have shown that 

 in protoplasm which has been deprived of its nucleus the 

 vital functions speedily become more or less deranged, and finally 

 cease altogether. Enucleated fragments of a cell or organism 

 fail to regenerate lost parts, and the apparent exceptions that have 

 been met with constantly turned out, on further investigation, to 

 be contaminated with nuclear influence. Enucleated fragments of 

 an amoeba are unable to excrete the substance which normally 

 enables these animals to cling to the substratum, and in other 

 organisms, although food may be ingested, the protoplasm seems 

 unable to digest and assimilate it. In plants Gerassimoff has 

 shown that cells containing chlorophyll but destitute of a nucleus 

 are usually unable to form starch, and are incapable of excreting a 

 limiting membrane over their free surfaces. 



On the other hand, allusion has already been made to the 

 transformations the nucleus may undergo in connection with the 

 secretory activity of glandular and other cells, and in this connec- 

 tion, no less than in that of regeneration, the nucleus may be 

 affirmed to preside over the metabolism of the protoplasm. 



The peculiar processes which are extruded by the nuclei of the 

 nutritive cells surrounding the ovum (Ophryotrocha), and the 

 curious simulation of the initial phases of mitosis met with in 

 secretory cells, can hardly be dissociated from the special functions 

 discharged by their nuclei. 



