100 THE HORSE. 



of the embryo, and of the yolk, which nourishes both, until the 

 vessels of the mother take upon themselves the task or, in ovip- 

 arous animals, till hatching takes place, and external food is to be 

 obtained. The ovum is carried down by the contractile power of 

 the fallopian tubes from the ovary to the uterus, and hence it does 

 not require automatic particles like the semen. 



6. THE EMBRYO, or young animal, is the result of the contact 

 of the semen with the ovum, immediately after which the sperm- 

 cell of the former is absorbed into the germ-cell of the latter. Upon 

 this a tendency to increase or " grow" is established and supported 

 at first, by the nutriment contained in the yolk of the ovum, until 

 the embryo has attached itself to the walls of the uterus, from 

 which it afterwards absorbs its nourishment by the intervention 

 of the placenta. 



7. As THE MALE AND FEMALE each furnish their quota to the 

 formation of the embryo, it is reasonable to expect that each shall 

 be represented in it, which is found to be the case in nature ; but 

 as the food of the embryo entirely depends upon the mother, it may 

 be expected that the health of the offspring and its constitutional 

 powers will be more in accordance with her state than with that 

 of the father ; yet since the sire furnishes one-half of the original 

 germ, it is not surprising that in external and general character 

 there is retained & fac-simile, to a certain extent, of him. 



8. THE OVUM OF MAMMALIA differs from that of birds chiefly 

 in the greater size of the yolk of the latter, because in them this 

 body is intended to support the growth of the embryo from the 

 time of the full formation of the egg until the period of hatching. 

 On the other hand, in mammalia the placenta conveys nourish- 

 ment from the internal surface of the uterus to the embryo during 

 the whole time which elapses between the entrance of the ovum 

 into the uterus and its birth. This period embraces nearly the 

 whole of the interval between conception and birth, and is called 

 uiero-gestation. 



9. IN ALL THE MAMMALIA THERE is A PERIODICAL " HEAT," 

 marked by certain discharges in the female, and sometimes by other 

 remarkable symptoms in the male (as in the rutting of the deer). 

 In the former it is accompanied in all healthy subjects by the de- 

 scent of an ovum or ova into the uterus ; and in both there is a 

 strong desire for sexual intercourse, which never takes place at 

 other times in them (with the single exception of the genus Di- 

 mana). 



10. THE SEMEN retains its fructifying power for some days, if it 

 is contained within the walls of the uterus or vagina, but soon ceases 

 to be fruitful if kept in any other vessel. Hence, although the 

 latter part of the time of heat is the best for the union of the sexes, 

 because then the ovum is ready for the contact with the semen, 



