26 



This author §scertainea in different woody plants that 

 wood cells couSid not attain their normal length tinder the 

 influence of artificial mechanical strain. To be sure, 

 the differences are not very great, in Fa gus silvatica 

 var, pendula. the wood cells of normal branches, for ex- 

 ample, bear to weighted ones, a proportion of 33.224 to 

 29.525, etc. 



To return finally to the statements made at the be- 

 ginning of this section,- it can be proved forthwith that 

 most of the examples of reduction in cell-size given here 

 also furnish examples of the second of the two modes of 

 grov;th described above. As examples of the first named mode 

 of those given here only the phenomena observed by Klebs 

 in Euastrum cone under our consideration,- as well as the 

 case of the plant which had been put into a plaster cast, 

 and which was studied by Hottes. 



C. DIFFSREHT1ATI0]SI OF THE CELLS AND TISSUES 



An organ attains its specific character through dis- 

 tinct processes of differentiation enacted in its cells 

 rather than by the number and volume of the cells composing 

 it. The cells attain a characteristic form by definitely 

 regulated or localized growth. Further, the histology and 

 physiology of the cell is determined by the formation of 

 living or lifeless cell contents, by thickening of the cell 

 walls, and by modification of its chemical composition. In 

 most cases, the formation of the cells of an organ differs 

 according to their position in it, so that finally the 

 matured organ is composed of elements of very different 

 kinds . 



In the study of arrested developments, those cases come 

 first under consideration in which the formative process 

 of individual cells stops prematurely, so far as form, 

 wall, or contents are concerned; secondly those in which 

 the cells of a tissue or an organ, varying in the same sense 

 from the normal procedure, are developed and produce a b.o- 

 mogenous tissue instead of vrell differentiated layers, 



1. ^SMATIOH OF THE CELL 



Both of the essential elements of the cell,- cytoplasm 

 31 and nucleus,- are excluded here from our discussion. As yet, 

 only very little has been definitely ascertained concern- 

 ing the structural peculiarities of cytoplasm and only in 

 isolated cases have v/e been half way instructed ontogen- 

 etically about the production of definite structural dif- 

 ferences. Conditions, so far as the nucleus is concerned, 

 are more favorable. In some ciliates (Oxytri chides) , in 

 the formation of nuclei, with two or more parts, or even 

 rosette-like, and in the distinctive formation of macro- 

 and micro-nuclei, v/e may See complicated' processes just as 

 in the phenomena visible in karyokinesis, the production 

 of which may doubtless be arrested or completely suppressed 

 by more or "less violent experimental interference. As is 

 well-known, it has been possible repeatedly to cause the 

 production of the more simple amitotic stages of cell- 

 division. 



