THE EMBKYO. 263 



the uterus, from which it afterward, absorbs its nourishment by 

 the intervention of the placenta. 



As the male and female each furnish their quota to the for- 

 mation 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 mo- 

 ther, 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, facsimile^ to a certain 

 extent, of him. 



The ovum or 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 eg^ until the period of hatch- 

 ing. On the other hand, in mammalia the placenta conveys 

 nourishment from the internal surface of the uterus to the em- 

 bryo 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 %dero-gestation. 



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. In the former it is accom- 

 panied 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. 



The semen retains its fructifying power for some days, if it 

 be contained within the walls of the uterus or vagina, but soon 

 ceases to be fruitful if kept in any otlier vessel. Hence, al- 

 though 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 

 uninjured until the descent of the ovum. 



The influence of the male upon the embryo is partly depen- 

 dent upon the fact, that he furnishes a portion of its substance 

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



