Human Physiology. 



259 



by means of the umbilical artery and veins. Through this 

 admirable organism the blood of the mother furnishes the 

 material from which is formed the blood of the child, and by 

 which it is nourished and built up. 



The human ovum, at its impregnation, is very small smaller 

 than the naked eye can distinguish. It is from the i-i2oth 

 to the i-i4oth of an inch in diameter. But from the moment 

 of fecundation, it grows with great energy. In a fortnight it is 

 of the size represented in Fig. 64. The foetus 

 of one month is an inch long; two months, two 

 inches and a half long; three months, five 

 inches; five months, six or seven inches; seven 

 months, eleven inches; eight months, fourteen 

 inches; nine months, eighteen inches. 



The interior structure of the ovum, and the 

 gradual development of the germ, embryo, and foetus, are best 

 explained and illustrated by reference to the larger eggs of 

 fishes and birds. The hen's egg may be taken as a model, and 

 when a hen is setting, or, more 

 humanely, when eggs are being 

 hatched by artificial heat, if 

 one be broken every.second or 

 third day, the whole develop- 

 ment may be watched with 

 great facility. Nature, it will 

 be seen, has prepared every- 

 thing, forgotten nothing, and 

 goes on in the formation of a 



Fig. 64. 



HUMAN OVUM 

 LAID OPEN. 



Fig. 65. IDEAL SECTION OF A 

 HEN'S EGG.* 



the right Fallopian tube ; the cavity of the uterus is almost completely occu- 

 pied by the ovum, e, e. Points of reflection of the decidua reflexa. f. 

 Decidua scrotina. g. Allantois. h. Umbilical vesicle, with its pedicle in 

 the umbilical cord. i. Amnion. k. Chorion ; between the two, the space 

 for the albumen. 



* The egg of the fowl is the type of all ova, and from its large size is easy 

 to study, a, a. Shell, b. Space filled with air to supply oxygen, c. Mem- 



