I06 BIOLOGICAL LECTURES. 



elementary parts, not differing from the others, arc capable of 

 separating themselves frovi the organism, and pursuing" an 

 independent growth, we may thence conclude that each of the 

 other elementary parts, each cell, is already possessed of power 

 to take up fresh molecules and grow ; and that, therefore, 

 every elementary part possesses a power of its oivii, an inde- 

 pendent life, by means of ivhich it zvould be cjiabled to develop 

 independently, \v the relations which it bore to external 



PARTS were but SIMILAR TO THOSE IN WHICH IT STANDS IN 



THE ORGANISM. The ova of animals afford us examples of 

 such independent cells, growing apart from the organism." 

 {I.e. p. 192). 



In these words of Schleiden and Schwann we see no vague 

 anticipation, but a clear statement, of the cell-standpoint of 

 to-day. The organism consists, morphologically, of cells, and 

 subsists, physiologically, by means of the "reciprocal action" 

 of the cells. Organization means cellular structure, and 

 ontogeny means cell-formation. " Der gleichc Elementarorgatiis- 

 mus ist es, der TJiiere und Pflanzen ziisammensetzt.'" (Schwann.) 



In this "double life," this "harmonious whole," this "recip- 

 rocal action" of "elementary organisms," this "operating 

 together in an unknown manner," we see the "cell-state" 

 theory, the "unknown principle of correlation," the "correlative 

 differentiation," the "cellular interaction " of current literature. 



Much as we have enlarged our knowledge of the cell, we 

 are still looking at the problems of life from the point of 

 view occupied by the founder of the cell-doctrine. The most 

 notable advances in cytology ha\e but tended to define and 

 emphasize the cell-standpoint. The discovery that all cells 

 arise by division of preexisting cells, neatly embodied in Vir- 

 chow's ma.xim, '' ouinis cclhila e cellnla'\- the extension and 

 verification of this maxim furnished by Gegenbaur in 1861, 

 in demonstrating the vertebrate egg to be a single cell ; and the 

 proof obtained during the last twenty years that the internal 

 processes of cell-division are fnndanientally the same in both 

 plajits and animals, — all these capital steps forward have 

 tended to magnify the importance of the cell as a universal 

 unit of structure. 



