CLASSIFICATION OF THE PRIMORDIA 97 



ance to traction of the epidermis is greater in the direction 

 NS than in the direction EW. 



It may happen that the division into intermediate segments 

 occurs in the direction of both axes ; this is observed in certain 

 AlgEe ; for instance. Ilea. (See N. WILLE in ENGLER und 

 PRANTL.) Here the segments x+t consist of four cells each, 

 and follow each other in both directions in the same way, being 

 united into a regular checker- work. ^ 



A more complicated segmentation isfound in the genus 

 Prasiola. (See N. WILLE in ENGLER und PRANTL.) Here 

 a specimen x is segmented into units x+x, which follow one 

 another according to both axes, NS and EW. Each unit 

 x+x consists of four units, x+2. 

 Each unit x+2 consists of four 

 units (cells), x + 2, (Fig. ii). 



In the three above examples 

 (onion. Ilea and Prasiola) the 

 individuals intermediate between 

 X and the cells are very small and 

 consist of a few cells. Very often 

 the segments are larger, the laws 

 of segmentation being the same. 



The (epidermal) scales of :the 

 lizards and other (not all) reptiles 

 are segments (individuals, units) 

 of a certain order produced by a 

 segmentation of the epidermis. 

 In the tail of a Mzard, for instance, the epidermis x is divided 

 into transversal (annular) segments x+x, which foUow each 

 other in the (longitudinal) direction NS.'^ Each segment x+x 

 is divided in its turn into multicellular units, x + 2 (scales), 

 which follow each other according to an axis EW. (This dis- 

 position is in a certain sense inverse of that observed in the 

 onion, in which the segments x+x are longitudinal.) 



In the elytra of the Coleoptera we find innumerable examples 

 of chess-board systems, altered in the most various ways, which 

 aU consist of multicellular units. One of the finest examples is 

 observed in Calosoma sycophania (and other species of the same 



ra3Si 



Fig. 10. — Ilea. (Schematic) 



• A similar segmentation is observed in certain forms of Pleurococcus vulgaris, 

 but here the society of cells has an ephemeral existence : the cells isolate 

 themselves after a certain time. 



^The segmentation of the epidermis of the tail of a lizard into annulcir 

 segments x + i is independent of the segmentation of the interior parts, 

 which finds its expression in the vertebral column (vertebras), the muscular 

 system, etc. The conception that the body of a vertebrate animal is divided 

 into segments is chiefly based upon the skeleton. In reality, two or even 

 several (more or less concentric) independently segmented systems exist. In 

 a similar way, two or more independent systems exist in the body of the 

 Articulates, the Annelids, etc. (See Triaxial System, § 88.) 



