THE CELL DOCTRINE. 29 



relations of cells as far as he well could without ap- 

 prehending the grand fact that the nucleated cell is 

 the fundamental expression of organic forms." 



SCHLEIDEN AND SCHWANN. 



It was reserved for Schwann to accomplish this 

 master stroke in observation and generalization, 

 through the intermediate results of Schleiden, with- 

 out whose observations on vegetable structures, 

 the true position of the cell would probably have 

 remained undetected for some time longer. Schlei- 

 den, in 1838, clearly pointed out the formation of 

 cells in vegetable structures, according to a single 

 and uniform method, and elaborated the theory of 

 development of which the cell was the unit, and 

 which Schwann immediately extended to animal tis- 

 sues. 



A formidable obstacle for some time in the way of 

 a law of development, applicable to animal and veg- 

 etable tissues, was the opinion, long entertained, that 

 the growth of animals whose tissues are furnished 

 with vessels is essentially different from that of 

 plants; an independent vitality being ascribed to the 

 elementary particles of vegetables growing without 

 vessels. So firmly was this believed, that the ovum 

 which exhibited undoubted evidences of an actual 

 vitality at one period of its growth, was said by all 

 physiologists to have had a plant-like growth. This 

 obstacle was removed in 1837, by Henle,* who showed 



* Henle, Symbolee ad Anatomiam vill. intest. Berol. : 1837. 



3* 



