40 VARIATION 



Passing backward in the same specimen, the i66th vertebra is 

 seen to be normal on the left but double and bearing two ribs on 

 the right, thus greatly crowding the ribs on that side. The i85th 

 vertebra is reported in the same condition, both being doubled 

 on the right side and single on the left (see Fig. 3). 



Following, Bateson l gives two examples of the reverse condi- 

 tion, namely with duplicity on the left side, and another with 



duplicity on the right, showing 

 clearly that meristic variation in 

 one side of a bilateral symmetry 

 may or may not involve the other 

 side. 



Ribs. Variation in the dorsal 

 region necessarily involves the 

 ribs. Aside from this, all evi- 

 dence goes to show that partial 

 division of the ribs is much more 

 common than is variation in the 

 number of vertebrae. In man, 

 for example, the cartilage is fre- 

 quently divided for a considerable 

 distance (1.5 in.) back from the 



FIG. 3. Meristic variation in vertebrae: ster num, often involving a real 

 double on right side. After Bateson , _ , .. . ir 



bifurcation of the rib itself. 



Homceotic variation in vertebrae and ribs. 2 These may be out- 

 lined as follows (all in man, except as noted) : 



I. Cervical resembling dorsal: backward homceosis. The dis- 

 tinguishing character of dorsal vertebrae is the bearing of ribs, 

 but this character is often assumed by neighboring cervical, 

 being common on the seventh and not unknown on the sixth. 

 Of fifty-seven cases examined by Struthers, forty-two showed 

 ribs on both sides and fifteen on one side only, showing a tend- 

 ency to preservation of symmetry. The completeness of develop- 

 ment ranges all the way from the merest rudiments (rare) to a 

 perfect rib connected by cartilage to the sternum (also rare), the 

 commonest form ending free or being joined by cartilage to the 

 first true rib. 



1 Bateson, Materials, etc., p. 105. 2 Ibid. pp. 106-128. 



