58 GENERAL SKETCH OF THE CELL 



primary conditions of development, and \vc have here, as I believe, 

 a clue to its determination.^ 



1. The Cell in Relation to the Multicellular Body 



In analyzing the structure and functions of the individual cell we 

 are accustomed, as a matter of convenience, to regard it as an inde- 

 pendent elementary organism or organic unit. Actually, however, 

 it is such an organism only in the case of the unicellular plants and 

 animals and the germ-cells of the multicellular forms. When we 

 consider the tissue-cells of the latter, we must take a somewhat dif- 

 ferent view. As far as structure and origin are concerned the tissue- 

 cell is unquestionably of the same morphological value as the 

 one-celled plant or animal ; and /;/ tJiis sense the multicellular body 

 is equivalent to a colony or aggregate of one-celled forms. Physi- 

 ologically, however, the tissue-cell can only in a limited sense be 

 regarded as an independent unit ; for its autonomy is merged in a 

 greater or less degree into the general life of the organism. From 

 this point of view the tissue-cell must in fact be treated as merely 

 a localized area of activity, provided it is true with the complete 

 apparatus of cell-life, and even capable of independent action 

 within certain limits, yet nevertheless a part and not a whole. 



I There is at present no biological question of greater moment than 

 the means by which the individual cell-activities are coordinated, and 

 the organic unity of the body maintained ; for upon this question 

 hangs not only the problem of the transmission of acquired charac- 

 ters, and the nature of development, but our conception of life itself. 

 Schwann, the father of the cell-theory, very clearly perceived this ; 

 and after an admirably lucid discussion of the facts known to him 

 ('39), drew the conclusion that the life of the organism is essentially 

 a composite ; that each cell has its independent life ; and that " the 

 whole organism subsists only by means of the reciprocal action of the 

 single elementary parts." ^ This conclusion, afterward elaborated by 

 Virchow and Haeckel to the theory of the "cell-state," took a very 

 strong hold on the minds of biological investig&tors, and is even now 

 widely accepted. It is, however, becoming more and more clearly 

 apparent that this conception expresses only a part of the truth, and 

 that Schwann went too far in denying the influence of the totality of 

 the organism upon the local activities of the cells. It w^ould of 

 course be absurd to maintain that the whole can consist of more than 

 the sum of its parts. Yet, as far as growth and development are con- 



^ Cf. pp. 384, 424. We should remember that the germ-cells are themselves epithelial 

 products. 2 Untersiichtingeii^ Trans., p. 1 81. 



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