94 THE QUANTITATI\^ METHOD IN BIOLOGY 



is obtained. This is often (not always) observed in Pleuro- 

 coccus vulgaris (Fig. 7).^ In the segments of a rectangular 

 system (for instance, Fig. 7, 5) a number of successive divisions 

 may take place. If divisions NS and EW occur in all the 



segments (cells), the 

 segmentations NS 

 and EW alternating 

 regularly, a rect- 

 angular system is 

 obtained, in which 

 all the segments are 

 regularly placed in 

 rows NS and rows 

 EW, as the squares of 

 a chess-board. I call 

 this a chess - board 

 system. 



Regular or almost 

 regular examples of 

 the chess - board 

 system are found in 

 certain Algse (for in- 

 stance, certain Ulva- 



FiG. 7. — Pleurococcus vulgaris. (Schematic' 

 {1-5) Successive states. (6) Irregular specimen 



cece), in the leaves of certain mosses (disposition of the cells), in 

 the disposition of the scales on the wings of numerous butterflies, 

 in the elytrae of a number of Coleoptera, in many sheUs, etc. 

 Examples of the chess-board system, more or less altered, are 



inmmierable. The knowledge of ^_ 



this system is important for the 

 investigation of the primordia of 

 plants. It is of the highest im- 

 portance in the animal kingdom. 



§ 78.— PRIMORDIA IN A 

 RECTANGULAR BIAXIAL 

 SYSTEM (CHESS-BOARD 



SYSTEM). — In a regular chess- 

 board system four primordia may 

 be distinguished : (i) the number 

 of segments in the longitudinal 

 direction A^5 ; (2) the number of ^^^ g.— Part of 



board system. 



t 



L regular chess- 

 (Schematic ) 



segments in the transversal direc- 

 tion EW ', (3) the dimension of 

 the segments in the direction NS (length) ; (4) the dimension of 



1 The system represented in Fig. 7, 5, is, of course, quite different by its origin 

 from the quadricellular coenobium represented in Fig. 3, p. 79. Between both 

 mechanical concordance probably exists. 



