CLASSIFICATION OF THE PRIMORDIA 



101 



separately in the direction of both axes. By the application 

 of this method to the description of shells and insects, of the 

 epidermis of plants, etc., an exact quantitative description of 

 the species will be rendered possible. It may be anticipated, 

 moreover, that inter- 

 esting similarities and 

 mechanical laws will 

 be discovered. 



§ 83. — ALTERA- 

 TIONS OF THE 

 CHESS-BOARD SYS- 

 TEM (continued). 

 DIFFERENTIA- 

 TION. — Differentia- 

 tion may be brought 

 about by various causes 

 and occurs in aU pos- 

 sible degrees. 



When a segment of 

 any system (uniaxial 

 or biaxial) is different 

 from the other seg- 

 ments by one or several 

 properties, we call it 

 differentiated.^ 



For instance, in the 

 epidennis (chess-board 

 system) of numerous 

 plants and insects cer- 

 tain cells are different 

 from the others because 

 they are transformed 

 into hairs or glands, or 

 because they contain a 

 colouring substance or 

 a cystohth. It seems 



6 



c sy c^ 6 a. 



-sy-sy, plane of symmetry 



a 



Fig. 13, 2. 



often as if the position of such a 

 differentiated unicellular segment (idioblast) was accidental. 



In many cases, however, a differentiation may be regarded 

 es a result of a gradation of a peculiar kind, and it may even be 

 sx'rmised that this is a general rule. 



In a typical example of gradation the variation of a property 

 p along an axis is gradual, without any sudden jump. I call 



1 Segmentation and differentiation , which are quite distinct notions, are 

 sometimes confounded under the term differentiation. 



Instead of saying, for instance, that an antenna of a certain insect is differ- 

 entiated into twelve joints, we ought to say . . . segmented . . . or . . . 



divided. . 



