548 THE PEOPLE'S FARM AND STOCK CYCLOPEDIA. 



8. The ovum of the 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 pla- 

 centa conveys the nourishment from the internal surface of the 

 uterus to the embryo during the whole time which elapses be- 

 tween 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 utero-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 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 con- 

 tact with the semen, yet if the semen reaches the uterus first, 

 it will still cause a fruitful impregnation because it remains 

 there (or in fallopian tubes) uninjured until the descent of 

 the ovum. 



11. The influence of the male upon the embryo is partly 

 dependent upon the fact that he furnishes a portion of its sub- 

 stance in the shape of the sperm-cell, but also in great measure 

 upon the effect exerted upon the nervous system of the mother 

 by him. Hence the preponderence of one or other of the par- 

 ents will, in great measure, depend upon the greater or less 

 strength of nervous system in each. No general law is known 

 by which this can be measured, nor is any thing known of the 

 laws which regulate the temperament, bodily or mental power, 

 color or conformation, of the resulting offspring. 



