Introduction 161 



culminated, as already stated, in the establishment of the 

 separate science now known as Cytology, in the foundation 

 and development of which workers in the fields of animal 

 and vegetable histology have co-operated. 



A second line of investigation, perhaps more strictly 

 anatomical, concerned itself with the morphology of the 

 tissues. Originally following the lines of Naegeli, and 

 looking for a morphological basis on which to classify cells 

 and their combinations, we find Hanstein claiming the 

 occurrence of histogenetic layers with a striking similarity 

 to the germinal layers of the animal embryo, and Van 

 Tieghem and his followers promulgating a coherent theory 

 of fundamental unity underlying the vascular structure of 

 the higher plants. 



We find, however, a third line diverging from this at 

 an early period. Physiological conceptions were grafted 

 on to the study of anatomy. While they were applied to 

 the cell itself as a dynamical machine, they were extended 

 more widely to the conception of the tissues. The adapta- 

 tions of the tissues to the environment on the one hand, 

 and the influence of the latter in modifying both their 

 arrangement and their structure on the other, came to 

 alter the direction of the purely anatomical research, in the 

 end no doubt greatly to its advantage. At the outset, 

 however, it introduced an element of confusion, for classi- 

 fication of cell complexes on the lines of function, before 

 the function itself was actually ascertained with certainty, 

 was no easy task. The performance of some particular 

 function by organs derived from different sources added 

 to the difficulty, and helped to obscure clearness of ideas. 

 For a long time there was confusion also between morpho- 

 logical and physiological conceptions, and the limitations 

 of the two were not properly distinguished. The work 

 which first directed attention especially to the physiological 

 interpretation of anatomical structure was Schwendener's 



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