^ CLASSIFICATION OF THE PRIMORDIA 107 



(2) Disorder may be a consequence of gradation. We have 

 seen, for instance (§ 84, first example), that in a leaf of a moss 

 gradation produces a displacement of the cells and a curvature 

 of certain axes : it is obvious that disorder may be brought 

 about at the same time. 



(3) In a chess-board system each segment (for instance, each 

 cell) is an individual which is more or less independent of the 

 adjacent individuals. ^ All the individuals are plastic (as any 

 living individual whatever). Therefore diversity in the con- 

 ditions of development (see below) brings about variation, for 

 instance, with regard to size and form, and this is a cause of 

 displacement and disorder. 



The development of the segments is influenced in various 

 ways by external causes within the limits of one system. For 

 instance, a small object may cover a part of the surface of a 

 system (a leaf of a moss, the epidermis of an animal or a 

 plant, etc.) and modify locally temperature and light ; certain 

 segments may be affected by a shock or by temporary contact 

 with any object, etc. Such causes, acting during the sensitive 

 period, may favour or thwart cell-division in certain segments 

 (Fig. 15), accelerate or slacken the growth of certain imits, a 

 consequence of such local modifications being disorder. 



When comparable parts are investigated in several species 

 in respect of disorder it is often seen that regularity prevails 

 in certain species and disorder in others. Therefore disorder 

 seems to be, in a certain degree, a property of certain species. 

 (Compare, for instance, the dorsal surface of the pronotum, 

 which is a chess-board system, in a series of species of 

 Car ah us.) 



It may be surmised that regularity prevails in the majority 

 of those cases in which the system or its segments are elongated 

 according to one of the axes [NS or EW), whereas disorder 

 is ordinarily observed when the system or its segments are 

 rather isodiametric. EXAMPLE : The epidermal cells of 



^This physiological independence may be easily demonstrated. In the 

 epidermis oi an onion bulb, for instance, certain cells which are weakened by 

 one or another cause (a shock, a short contact with a hot or a very cold object, 

 etc.) may be killed by a certain poison, whereas the adjacent healthy cells are 

 not visibly influenced. If the poison is a plasmolysing substance (for instance, 

 KNO3 at 10 per cent.), the healthy cells are plasmolysed ; in the weakened 

 cells only the wall of the vacuole (tonoplast) is plasmolysed and separated 

 from the exterior part of the protoplasma (including the nucleus) which is 

 killed. In cells which are still more weakened the whole of the living con- 

 tents of the cell is killed at once by the mentioned poison without any trace 

 of plaismolysis. 



If a small quantity of eosine (purissima) is added to the plasmolysing 

 solution the above differences between adjacent cells become still more evident, 

 since the killed parts are stained, the living (plasmolysed) parts being colour- 

 less (E. VERSCHAFFELT). This question has been thoroughly investigated 

 by A. J. J. VANDEVELDE. 



