292 SOME PROBLEMS OF CELL-ORGANIZATION 



The visible organs of the cell fall under two categories, according as 

 they are merely temporary structures, formed anew in each successive 

 cell-generation out of the common structural basis, or permanent struc- 

 tures whose identity is never lost, since they are directly handed on by 

 division from cell to cell.^ To the former category belong, in general, 

 such structures as cilia, pseudopodia, and the like ; to the latter, the 

 nucleus, perhaps also the centrosomes, and the plastids of plant-cells. 

 A peculiar interest attaches to the permanent cell-organs. Closely 

 interrelated as these organs are, they nevertheless have a remarkable 

 degree of morphological independence. They assimilate food, grow, 

 divide, and perform their own characteristic actions like coexistent but 

 independent organisms, of a lower grade than the cell, living together 

 in colonial or symbiotic association. So striking is this morphological 

 and physiological autonomy in the case of the green plastids or chro- 

 matophores that neither botanists nor zoologists are as yet able to dis- 

 tinguish with absolute certainty between those that form an integral 

 part of the cell, as in the higher green plants, and those that are 

 actually independent organisms living symbiotically within it, as is 

 probably the case with the yellow cells of Radiolaria. Even so 

 acute an investigator as Watase ('93, i) has seriously propounded the 

 view that the nucleus itself — or rather the chromosome — should be 

 regarded as a distinct organism living in symbiotic association with 

 the cytoplasm, but having had, in an historical sense, a different origin. 

 This rather fantastic view has not found much favour, and even were 

 it true would teach us nothing of the origin of the power of division, 

 which must for the present be taken as an elementary process forming 

 one of the primary data of biology. Yet we may still inquire whether 

 the power of division shown by such protoplasmic masses as plastids, 

 chromosomes, centrosomes, nucleoli, and nuclei may not have its root 

 in a like power residing in ultimate protoplasmic units of which they 

 are made up. Could we accept such a view, we might much more 

 easily meet some puzzling cytological difficulties. For under this 

 assumption the difference between transient and permanent cell- 

 organs would become only one of degree, depending on the degree of 

 cohesion between their structural components ; and we could thus con- 

 ceive, for example, how such a body as a centrosome might form, per- 

 sist by division for a number of generations, and finally disintegrate. 

 In connection with this it may be pointed out that even such a typical 

 permanent organ as the nucleus does not persist as such during the 

 ordinary form of division ; for it loses its boundary and many of its 

 other structural characters, becoming resolved into a group of sepa- 

 rate chromosomes. What persists is here not the structural unit, but 

 the characteristic substance which forms its essential constituent, and 



^ Cf. footnote, p. 30. 



