6. The horse, then, bears young at intervals, as I have observed; but the ass is not subject to intervals. Some mares are quite barren, and others, though they conceive, yet do not produce their young; and they give as a reason for this, that upon dissection the fœtus was found to contain other reniform bodies round the kidneys, so that it appeared to have four kidneys. As soon as the mare has foaled, she eats the chorion, and bites from the head of her foal the substance called hippomanes. In size this substance is somewhat less than a dry fig. Its form is flat and round, and its colour black. If any person is at hand to take it before the mare, and she smells it, the scent renders her wild and mad. For this reason it is sought after and collected by poisoners. If an ass copulates with a pregnant mare, the pre-existing fœtus is destroyed. Those who keep herds of horses do not place a leader over them, as they do over oxen, for they are not naturally stationary, but active and wandering. CHAPTER XXIII. 1. The male and female ass begin to copulate at thirty months old, and shed their first teeth at the same period. They lose their second pair of teeth six months afterwards, and their third and fourth in the same way. These fourth teeth are called the marking teeth. Sometimes the ass has become pregnant and brought up its young at a year old. The she ass parts with the semen after coition, if she is not prevented; and therefore, immediately after coition, they beat her and drive her about. She foals in the twelfth month, and generally produces one foal, for this is their nature, though cases of twin births have occurred. If an ass mounts upon a mare, he destroys her fœtus, as I observed before. But the horse does not destroy the fœtus of the ass, if the mare has been impregnated by a he ass. 2. The pregnant female has milk at the end of ten months. After parturition, she will admit the male on the seventh day, and is very easily impregnated at that period. She will also receive it afterwards. If she does not produce young before losing her marking teeth, she can never be impregnated all the rest of her life. She does not like men to be witnesses of her parturition, nor will she produce her young in the day time; but when it is dark she retires, and so produces her young. She continues to procreate during her whole life, if she has begun before losing her marking teeth. The ass lives more than thirty years, and the female longer than the male. When a horse copulates with an ass, or a he ass with a mare, abortion is more frequent than between congeners, a horse with a mare, or two asses together. When the horse and ass are mixed together, the period of gestation follows from the male parent. I mean to say that it takes the same time as if the parents had been congeners; but in size, form, and strength the produce of their union generally resembles the female parent.