Fig. 7. — Pleurococcus 

 (1-5) Successive states. 



vulgaris. (Schematic.) 

 (6) Irregular specimen 



94 THE QUANTITATIVE IMETHOD 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 Algje (for in- 

 stance, certain Ulva- 

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

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

 in the elytrse of a number of Coleoptera, in many shells, etc 



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

 innumerable. 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 pj^ g 

 segments in the transversal direc- 

 tion EW ; (3) the dimension of 

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



• The system represented in Fig. 7, 5; is, of course; quite dijBEerent by its origin 

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

 mechanical concordance probably exists. 



Part of a regular chess- 

 board system. (Schematic) 



