DEVELOPMENT OF THE HORSE. 13 



tri-dactylous, there can no longer be any doubt as to its belonging 

 to the great horse family. The limbs are better formed, and, 

 compared with the tail, relatively longer than at the end of the 

 fifth week. The embryonic sac {a, h, c, d), still ovoid, measures 

 three inches in the one direction, and over two in the other, 

 and the allantois {all.) has fused with the greater extent of its 

 inner surface. The yolk sac (y.s.) has still about the same 

 capacity as at the end of the fourth week, but the absorbing area 

 (a, b, c) is only about half the size. The girdle (t.g.), equatorial 

 in position at the end of the fifth week, now lies near the pole 

 occupied by the absorbing area. 1 From the absorbing area being 

 comparatively small, and the special fixing structures occupying 

 one pole, I am inclined to think that, about the end of the sixth 

 week, the whole of the embryonic sac might be easily detached. 

 Were it an opossum embryo, preparations would soon be made 

 for its birth. The horse may not have quite forgotten this 

 ancestral habit. About the end of the sixth, as at the end of the 

 third week, the whole reproductive system is in all probability 

 in a somewhat excitable condition. All the physiological 

 changes which occur during oestrum are likely to supervene 

 in a more or less pronounced form about the end of the third 

 and again at the end of the sixth week. In other words, the 

 habit which the nervous and other systems have of becoming 

 periodically excited is not apparently quite thrown off for some 

 weeks after a successful service. In some mammals the nearly- 

 ripe eggs found in the ovary after development has started are 

 said to be absorbed ; but in the mare one or more eggs may be 

 matured and discharged several weeks after she has settled. There 

 is a case on record of a mare bringing forth twins, a foal and a 

 mule. She was presented to a jackass fifteen days after being 

 served by a horse, The escape of ova (ovulation) is accompanied 

 with an extra rush of blood to the ovaries and to the uterus, which 

 implies an excited condition of the nervous apparatus of these 

 organs, increased secretion in the uterine glands, and more or 



weeks, embryos from small mares are not likely to differ much, if at all, in 

 size, from embryos taken from large mares. My five smallest embryos (four to 

 eight weeks) are from mares from 14*2 to 15 hands high. 



1 In the only six-weeks' specimen I have seen, this girdle was narrow and 

 broken up into short links or segments, as if in process of disintegration. 



