Fig. 561. - Mare and New-born Foal.

Fig. 562. - Hind Limbs and Tail (four weeks).

Fig. 563. - Hind Limbs and Tail (six weeks).

Fig. 564. - Hind Limbs and Tail (eight weeks).

In figs. 562, 563, and 564 the growth of the hind-limbs and tail from 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 development 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 ancestors 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 Hermit. From the twenty -fourth week onwards the limbs grow faster than the trunk. In front the increase in growth is mainly beyond the wrist-joint, while in the hind-limb it is chiefly beyond the knee-joint or stifle. In both fore- and hind-limbs the increase is greatest for some time in the middle metacarpals (fig. 565, III) and metatarsals - i.e. in the fore and hind cannon bones. In the case of Hermit 1 (the 1867 Derby winner), when the humerus is taken as equal to 100, the third metacarpal is equal to 76.1. In a seven-months fetus, however, with the humerus equal to 100, the third metacarpal may be over 90, while at nine months it may be over 110, and at birth 130 - i.e. relatively well-nigh twice the length of the third metacarpal in Hermit. A similar rate of growth characterizes the third metatarsal bone. This increase in the cannon bones during the second half of the period of gestation explains to a large extent the great length of the foal's legs at birth; it also accounts for the fact that the cannon bones - the bones considered of so immense importance in all kinds of horses - increase but little in length after birth. In the case of a thoroughbred, e.g., the third metacarpal appears only to increase 3 cm. (barely l¼ inch) after birth, 2 while the main bone of the fore-arm (the radius) often increases 9 cm. (3 ½ inches), or nearly three times as much as the front cannon bone.

1The skeleton of Hermit is preserved in the Royal Veterinary College Museum, Camden Town, London. Eclipse's skeleton is in the Royal College of Veterinary Surgeons Museum, Red Lion Square, London.

2 Though the cannon bones may only increase 1 inch in length after birth, they may increase 3 or even 4 inches in circumference in a 14-hands horse.

Fig. 565. - Fore-limb (twenty-three weeks), natural size.

From the twenty-fourth to the beginning of the thirty-fourth week, horse embryos often increase at the withers from 32.5 cm. to 54 cm., while from the elbow onwards the increase is from 21 cm. to 34.5 cm., and from the point of the hock to the tip of the hoof the increase is 10.5 cm. - i.e. from 15 cm. to 25.5 cm.

At the fortieth week the embryo is about 86 cm. in length, the height at the withers being 76 cm.; from the elbow to the point of the hoof the distance is 52-53 cm., and from the hock 40-42 cm.