292 BREEDING 



the fourth to the end of the eighth week is represented five times natural 

 size. It will be observed that as the limbs increase in length and com- 

 plexity the tail gets relatively shorter. In the Eocene " fossil horses " the 

 tail, at first long enough to trail on the ground, gradually dwindled as the 

 heels (hocks) were raised. In becoming relatively shorter during develop- 

 ment the tail may be said to repeat the ancestral history. It may here be 

 added that up to near the time of birth there are only long hairs at the 

 end of the tail, from which it may be inferred that in the Miocene an- 

 cestors of the horse the tail resembled that of the living asses and zebras. 



During the eighth week the embryo nearly doubles its length, but 

 during the three following weeks there is an increase in bulk rather than 

 in length. At the end of the eleventh week the total length is 10 "2 cm., 

 the length from the elbow being 3 '15 cm., from the withers 5 "2 cm., and 

 from the hock 2 '4 cm. 



From the eleventh to the fifteenth week the embryo again more than 

 doubles its length, and increases considerably in weight. At the end of 

 the fifteenth week the length is 23 cm., the height at withers 14 cm., the 

 length from the elbow 8 '8 cm., and from the hock 6 "6 cm. Again, from 

 the fifteenth to the twenty -fourth week the total length is nearly doubled, 

 while the length of the limbs is more than doubled. At the beginning of 

 the twentieth week the total length is 28 cm., the height at the withers 

 19 '5 cm., the length from the elbow 12'3 cm., and from the hock 9 '2 cm. 

 By the time the twenty-fifth week is reached the total length of the young- 

 horse is 43 '5 cm., the height at the withers 32 '5 cm., the length from the 

 elbow 21 cm., and from the hock 15 cm., the circumference below the knee 

 being 3 cm. As already mentioned, before the middle of gestation is 

 approached i.e. prior to the twenty-fourth week the bones of the limbs 

 are as nearly as possible of the same relative length as in the adult. 



This fact is best brought out by comparing the limb bones of a five- 

 and-a-half-months embryo with the corresponding structures in the adult. 

 In the thoroughbred horse Hermit the humerus measured 33 '5 cm., the 

 radius 37 '5 cm., and the third metacarpal 25 '5 cm. In a twenty-three- 

 weeks embryo the humerus is 6 '5 cm. in length, and the radius 7 '3 cm. 

 i.e. the radius bears exactly the same relation to the humerus as in the case 

 of Hermit. Again, the middle (III) metacarpal in a half-time embryo, to 

 agree with the corresponding bone in Hermit, should measure 5'0 cm.; its 

 actual length is 5 '5 cm., i.e. it is already 5 mm. relatively longer than in 

 Hermit; but this is more than counterbalanced in the twenty-three-weeks 

 embryo by the phalanges being relatively shorter. In figs. 565 and 566 

 the bones of the fore-limb of a twenty-three-weeks foetus, the radius and 

 third metacarpal bear nearly the same relation to the humerus as in 



