162 THE HORSE 



are intended to co-operate with the germ-ceU of the ovum in forming the 

 embryo. 



5. THE OVUM consists of the germ-cell, intended to form part of the 

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

 mother take upon themselves the task ; or, in oviparous 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 forma- 

 tion of the embryo, it is reasonable to expect that each shall be repre* 

 sented in ife, which is found to be the case in nature ; but as the food oi 

 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 externals 

 and general character there is retained a 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 nourishment 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 

 ultra-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 descent 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 Dimana). 



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 

 fruitfifl if kept in any other vessel. 1 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, yet if the semen reaches 



1 It has been conveyed in a suitable vessel and at the natural temperature of the body to 

 great distances, and extending over many hours without losing its fecundating properties. 

 An instrument known as the certes impregnate! 1 has been employed successfully, first in 

 America and subsequently in England. 



