98 THE QUANTITATIVE METHOD IN BIOLOGY 



genus). An elytrum of this handsome beetle is divided into a 

 number of units (longitudinal ribs), which foUow each other in 

 the direction EW. Each rib is segmented in its turn into 

 multicellular quadrangular segments x+2, which follow each 

 other according to an axis NS.^ On the whole, the disposition 

 of the segments recalls that of the cells in the epidermis of an 

 onion (Fig. 9). (Mechanical concordance.) 



An unlimited number 

 of various examples are 

 observed in the shells 

 of the moUuscs. 



These objects have been 

 hitherto described in a rather 

 empirical way. When they are 

 looked upon as being chess- 

 board sj'stems, governed 

 by the principles expounded 

 in this book, they afford a 

 very interesting and fascin- 

 ating subject of investigation 

 and measurement.^ In order 

 to realize the importance of 

 the shells, which have been 

 rather neglected by the mor- 

 phologists (and disdained by 

 many biologists), one may 

 consider their importance 

 for the study of plasticity 

 (influence of the conditions 

 of existence), zoological 



Fig. II. — Prasiola. (Schematic) 



geography, oceanography, palaeontology (here investigation is limited to the 

 shells) and geology. (See §89.) 



§81.— LIMITS OF THE SEGMENTS. VEXILLARY 

 MARKS. — In the preceding examples the limits between the 

 segments are distinct. These limits, however, may get more or 

 less obUterated or disappear completely. Even in such cases 

 it is often possible to discover the disposition of the segments 

 and to count them by means of certain vexillary marks. 



FIRST EXAMPLE: In certain parts of the human skin, 

 for instance on the palm of the hand, we observe a number of 

 parallel (somewhat curved and irregular) furrows,^ separated by 

 intervals of about one half-miUimetre. Each ribbon-like seg- 

 ment between two furrows is an individual x+i (the epidermis 

 being x) comparable to the longitudinal segments x+i in 

 Fig. 9 and to the annular segments of the tail of a lizard. In 



1 In this description, only the part of the elytrum comprised between its 

 proximal border (sutuia) and the so-called series umbilicata (raphe) is taken 

 into account. A number of details are passed over in silence. 



" I have measured a number of primordia in many species. My notes are 

 not within my reach. See further. 



^ These furrows are easily distinguishable from the very variable oblique 

 wrinkles of the skin. 



