CHAPTER II 

 THE DIFFERENTIATION OF THE TISSUES 



THE grouping of cells to constitute tissues, and the 

 recognition that the tissues in turn are grouped or asso- 

 ciated in various ways to constitute tissue systems, occupied 

 the attention of many anatomists after 1860. We have 

 seen that just at that time a new impetus had been given 

 to the study, and its main direction materially influenced, 

 if not altogether changed, by the new point of view trace- 

 able to the idea of adaptation to environment. In the 

 absence of such an idea morphological conceptions only 

 had prevailed, and they were of but recent introduction. 

 Unger's classification of 1855, good as it was for the time, 

 was almost empirical, and Naegeli was really the first to 

 treat the subject on a scientific basis. In his writings we 

 find Morphology taken definitely as underlying and supply- 

 ing the principles of classification. Promulgated as it was 

 in 1858, it may be taken to open the period now under 

 review. 



A work of a very important nature was published by 

 Sanio in 1863. To many present-day readers the name of 

 Sanio is chiefly connected with the discovery of his ' thicken- 

 ing ring '. This, however, constituted but a small part of 

 the work now before us. His treatise was the first to put 

 forward a proper conception of the only partially under- 

 stood process of the secondary thickening of the axis of 

 the Dicotyledons and Conifers. Sanio showed that it con- 

 tains a central core surrounded by a circumferential zone, 

 though the delimitation between them varies a good deal 

 in distinctiveness. In the region outside the central core 



