394 CHARLES R. STOCK ARD 



accompanied by a fully expressed short screw-tail (see 918$ 

 in plates 19 and 20). 



The skeletons of the right front limbs in plate 80 are also 

 of some interest in the present connection. All these show 

 achondroplasia of the long bones, but are heterozygous for 

 the condition, since they arose from the backcross of the 

 Fj hybrid on the long legged bulldog parent. We have pre- 

 viously pointed out that the dominant factor for achondro- 

 plasia in the extremities gives a more severe shortening in 

 association with bulldog quality bone than with hound bone. 

 It is fair to suppose that breed quality of bone may be 

 dependent upon a large number of factors in the genetic 

 constitution of the animal. It is further probable that in- 

 dividuals homozygous for the double factor recessive screw- 

 tail condition derived from the bulldog parent will approach 

 more nearly the genetic constitution necessary for the bull- 

 dog bone quality than do heterozygous individuals with long, 

 straight tails, since the former carry certain bulldog chromo- 

 somes which the latter necessarily lack. In view of these 

 facts, it is of interest to find that the leg skeleton accompany- 

 ing the long-straight tail (fig. 1) is less shortened than the 

 three others. This leg skeleton is probably nearer the basset- 

 hound bone in quality than are those accompanying the 

 short-bent and screw-tails. The more complete the bulldog 

 constitution, the shorter will be the leg. 



The fusion and shortening or abbreviation of caudal verte- 

 brae bringing about a reduction in tail length is separated 

 genetically from the vertebrae deformity which produces 

 bending and twisting of the tail. Although these phenomena 

 are two only slightly different distortions in cartilage growth, 

 they are genetically separate and independently expressed. 



PLATE 80 



EXPLANATION OP FIGURES 



Further details of tail length and structure in the backcross bulldog-basset- 

 hound-bulldog hybrids shown in plate 79, and a comparison of the tail and leg 

 skeletons of these hybrids. 



1 1481$. 2 1479 <?. 3 1478^. 4 1482$. 



