1 8 THE MAIN CURRENTS OF ZOOLOGY 



Schwann (Fig. 3) in an extensive paper (215 octavo 

 pages, with four plates), published in 1839 first em- 

 ployed the term " cell- theory ' and explained its 

 meaning. This treatise, which is a biological classic, 

 was his famous Microscopical Researches regarding 

 the Accordance in Structure and Growth of Animals and 

 Plants (Mikroscopische Untersuchungen uber die 

 Uebereinstimmung in der structur und dem Wach- 

 sthum der Thiere und Pflanzen). 



Schwann's writing in the Microscopical Researches 

 is clear and philosophical, and is divided into three 

 sections, in the first two of which he confines himself 

 strictly to descriptions of observations, and in the 

 third part of which he enters upon a philosophical 

 discussion of the significance of the observations. 

 He comes to the conclusion that: "The elementary 

 parts of all tissues are formed of cells in an analogous, 

 though very diversified manner, so that it may be 

 asserted that there is one universal principle of devel- 

 opment of the elementary parts of organisms, how- 

 ever different, and that this principle is the formation 

 of cells." 



It was in this treatise also that he introduced the 

 term cell- theory as follows: "The development of the 

 proposition that there exists one principle for the 



