290 



BREEDING 



The smallest horse embryo in my collection measures 7 mm. just 

 over ^ inch. This, a twenty or at the most twenty-one days embryo, is 

 somewhat fish-like in form, but quite limbless. Soon after the end of the 

 third week limbs appear in the form of minute buds. At the end of the 

 fourth week (fig. 562) they are easily recognized, and by the end of the 

 fifth week they are 2 mm. in length; at the end of the fourth week a horse 



Fig. 561. Mare and New-born Foal 



embryo measures 10 mm., and is not unlike a human embryo of the same 

 age; by the end of the fifth week it is 5-6 mm. longer. At first the limb 

 buds are simple paddle-like, structureless outgrowths, but during the fifth 

 week rudiments of the skeleton appear; while during the sixth week they 

 are so rotated and flexed that the position of the elbow and wrist (com- 

 monly called the " knee ") can be made out in the fore-limb, and in both 

 fore- and hind-limb there are indications of three digits (2-4). Even at 

 the end of the sixth week, when the embryo is 2 cm. in length, the fore- 

 limbs only measure 4 mm., and the hind-limbs are but little longer (fig. 



