CLASSIFICATION OF THE PRIMORDIA 95 



the segments in the direction EW (breadth). These very simple 

 notions are applicable, for instance, on the quantitative investi- 

 gation of the scales on the wings of the butterflies, taking 

 socially equivalent segments in the wings of all the spieces of 

 a genus or a family, 



§79.— ALTERATIONS OF THE CHESS-BOARD SYSTEM. 



— The typical chess-board system (Fig. 8) may be modified along 

 several lines. This results in numerous deviations and com- 

 phcations, many of which have been described as isolated facts, 

 or regarded as adaptations or mere curiosities.^ All these 

 modifications, however various and complicated they may be, 

 ought to be looked upon as states of equilibrium. Adopting 

 this view, we may regard them all from one standpoint — in a 

 similar way, as the mineralogists have brought together into a 

 whole the various deviations and complications of innumerable 

 crystals. 



In order that we might be able to analyse the primordia of a 

 given chess-board system, we want, above all, to know the direc- 

 tion of the axes. This is almost always easily found : we may 

 take as axis NS the longitudinal axis of the object under con- 

 sideration. Sometimes it is more convenient to take an anti- 

 clinal (radial) or a pericUnal (tangential) direction as axis NS. 

 In certain cases, the choice of the primary axis is arbitrary 

 for instance, in Pleurococcus (Fig. 7, 5), in which it is impossible 

 to discover any difference between the axes NS and EW. 



I want to recall that any axis whatever is a line of equilib- 

 rium, segmentation (or cleavage), differentiation and gradation : 

 therefore the axes are the lines of orientation [ordonnance] of 

 all the primordia of a biaxial system. 



In general, the primordia of a chess-board system may be 

 classified according to the scheme in § 72 (supplement in § 73, 

 p. 89). (See also §§ 57, 60.) 



Taking the above principles into account and starting from 

 the regular chess-board system (Fig. 8, p. 94), the alterations of 

 this system may be classified in the following way ^ : — 



I. A chess-board system (segment, unit or individual x) may 

 be divided into groups of cells which are individuals or units 

 x+i intermediate between the whole system x and its simple 

 components or cells x+2. In a similar way the pluricellular 

 units x-\-i may be divided into pluricellular units x+2, the cells 

 being a;+ 3, etc. (See § 80.) 



1 Numerous examples are found in the specific descriptions of shells, insects; 

 fishes, reptiles, etc. 



2 The following classification is probably incomplete : it ought to be regarded 

 as being a first attempt to set in order a number of facts which have been 

 hitherto overlooked or described in a fragmentary way. 



