THE NATURE OE CELL-ORGAXS 211 



A. The Nature of Cell-organs 



The cell is, in Briicke's words, an cloncntary organism, which may 

 by itself perform all the characteristic operations of life, as is the case 

 with the unicellular organisms, and in a sense also with the germ- 

 cells. Even when the cell is but a constituent unit of a higher grade 

 of organization, as in multicellular forms, it is no less truly an organ- 

 ism, and in a measure leads an independent life, even though its 

 functions be restricted and subordinated to the common life. It is 

 true that the earlier conception of the multicellular body as a colony 

 of one-celled forms cannot be accepted without certain reservations.^ 

 Nevertheless, all the facts at our command indicate that the tissue- 

 cell possesses the same morphological organization as the egg-cell, or 

 the protozoan, and the same fundamental physiological properties as 

 well. Like these the tissue-cell has 'its differentiated structural parts 

 or organs, and we have now to inquire how these cell-organs are to 

 be conceived. 



The visible organs of the cell fall under two categories according 

 as they are merely temporary structures, formed anew in each suc- 

 cessive cell-generation out of the common structural basis, or per- 

 manent structures 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, probably also the centrosome, and 

 the plastids of plant-cells. A peculiar interest attaches to the per- 

 manent cell-organs. Closely inter-related as these organs are, they 

 nevertheless have a remarkable degree of morphological indepen- 

 dence. 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 chromatophores that 

 neither botanists nor zoologists are as yet able to distinguish with 

 absolute certainty between those that form an integral part of the 

 cell, as in the higher green plants, and those that are actually inde- 

 pendent organisms living symbiotically within it, as is probably the 

 ca.se with the yellow cells of Radiolaria. Even so acute an investi- 

 gator as Watase ('93, i) has not hesitated to regard the nucleus 

 itself — or rather the chromosome — as a distinct organism living in 

 symbiotic association with the cytoplasm, but having had, in an his- 

 torical sense, a different origin. It is but a short step from this con- 



1 Cf. p. 41. 



