THE HISTORY OF THE CELL-THEORY 5 



anatomy of animals was, in spite of the greater difficulty of 

 observation, immediately placed upon the same plane as that of 

 plants. 



Two circumstances contributed to the rapid and brilliant result 

 of Schwann's observations. In the first place Schwann made the 

 greatest use of the presence of the nucleus in demonstrating the 

 animal cell, whilst emphasizing the statement that it is the most 

 characteristic and least variable of its constituents. As before 

 mentioned, this idea was suggested to him by Schleiden. The 

 second, no less important circumstance, is the accurate method 

 which Schwann employed in carrying out and recording his obser- 

 vations. As the botanists by studying undeveloped parts of 

 plants traced the development of the vessels, for instance, from 

 primitive cells, so he, by devoting especial attention to the history 

 of the development of the tissues, discovered that the embryo, at 

 its earliest stage, consists of a number of quite similar cells ; he 

 then traced the metamorphoses or transformations, which the cells 

 undergo, until they develop into the fnlly-formed tissues of the 

 adult animal. He showed that whilst a portion of the cells retain 

 their original spherical shape, others become cylindrical in form, 

 whilst yet others develop into long threads or star-shaped bodies, 

 which send out numerous radiating processes from various parts 

 of their surface. He showed how in bones, cartilage, teeth, and 

 other tissues, cells become surrounded by firm walls of varying 

 thickness; and, finally, he explained the appearance of a number 

 of the most atypical tissues by the consideration that groups of 

 cells become, so to speak, fused together ; this again is analogous 

 to the development of the vessels in plants. 



Thus Schwann originated a theory which, although imperfect 

 in many respects, yet is applicable both to plants and animals, and 

 which, farther, is easily understood, and in the main correct. 

 According to this theory, every part of the animal body is either 

 built up of elements, corresponding to the plant cells, massed 

 together, or is derived from such elements which have undergone 

 certain metamorphoses. This theory has formed a satisfactory 

 foundation upon which many further investigations have been 

 based. 



However, as has been already mentioned, the conception which 

 Schleiden and Schwann formed of the plant and animal element was 

 incorrect in many respects. They both defined the cell as a small 

 vesicle, with a firm membrane enclosing fluid contents, that is to say, 



