58 



GENERAL SKETCH OF THE CELL 



primary conditions of development, and we have here, as I believe, 

 a clue to its determination. ^ 



I. The Cell in Relation to the Multicellular Body 



In analyzing the structm-e 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 in tins 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 Hfe 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 hmits, yet nevertheless a part and not a whole. 



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 Hfe 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 investigators, 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 would 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- 



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

 products. * Untersuchungen, Trans., p. 181. ' 



