MERISTIC VARIATION 65 



saw one specimen of a leglike appendage growing from the left 

 side of the neck of a calf near the point of the shoulder. The 

 leg was not more than two thirds the usual length, and was 

 twisted and functionless, though it terminated with a hooflike 

 growth. 1 



Extra legs are common in insects, sometimes throughout their 

 entire length, sometimes doubling at the femur (see Fig. 16). 



SECTION III MERISTIC VARIATION AND BILATERAL 



SYMMETRY 



Meristic variation among paired organs and those standing 

 singly on the median line throws no little light upon the nature 

 of bilateral symmetry and also incidentally upon the manner of 

 variation. 



Speaking generally, paired organs may double on either side 

 separately or on both sides (digits, legs, wings, etc.), or they may 

 unite into a single organ with its axis on the median line (horse- 

 shoe kidney). 



Most of the examples of meristic variation already given are 

 of repetition in paired organs in bilateral symmetry. It remains 

 to call attention to the opposite condition, the fusion of a pair 

 into a single organ standing on the median line : 



1. A good example of this is that of a roebuck having the horns com- 

 pounded for fully half their length into a single " beam " standing on the 

 middle line 2 (see Fig. 17). 



2. A honeybee 3 having the two compound eyes united into one at the top 

 of the head with no groove or line of division between them. 4 



3. Posterior ends of kidney united (in man), forming a horseshoe kidney 

 with three renal arteries on each side. This case is in sharp contrast to 

 Bateson's No. 407, with a single large kidney on the left and two smaller, 

 one below the other, on the right. 



1 This specimen is described from memory, as it was seen before these phe- 

 nomena were matters of personal interest. 



2 Bateson, Materials, etc., p. 460. 



3 Ibid. p. 461. 



4 The compounding of eyes has already been mentioned. It apparently occurs 

 only in insects, but is a good example of the development of highly differentiated 

 tissue in abnormal situations, illustrating not only meristic variation but functional 

 variation as well. 



