Human Physiology. 263 



The human ovum is too minute for observation, and the 

 opportunities for such observation are necessarily rare. Up to 

 the seventh day after impregnation nothing is visible to the 

 naked eye; on the tenth a semi-transparent greyish flake; 

 on the twelfth a vesicle, size of a small pea, rilled with fluid, in 

 which swims an opaque spot, the embryo, which with its two 

 membranes, waters, &c., weighs about one grain; on the thir- 

 teenth day it resembles a worm bent together, and stretched 

 out is nearly half-an-inch long. In the seventh week bone 

 begins to form in the lower jaw and clavicle; the brain, eye, 

 and ear are growing more perfect; the heart is getting its 

 proper form; the liver is very large, and an active organ; but 

 the lungs, which are not needed until birth, are mere sacs, and 

 the trachese delicate threads. At from sixty to seventy days 

 all parts are rapidly growing; the eyes enlarge, the lids are 

 visible, the nose prominent, the external ear formed, and the 

 heart fully developed. At three months the eye-lids are dis- 

 tinct, the lips drawn together, the larger vessels carry red blood, 

 fingers and toes well denned, and muscles begin to be formed. 

 At four months the intestines are covered in by the abdominal 

 muscles, the lungs have grown and are even susceptible of a 

 slight dilatation. The skin begins to form, and the place of 

 the nails is visible. There is also some excretory action in 

 the intestines. At six months down appears on the head, fat 

 begins to be deposited. At seven months the foetus is fully 

 formed, and, if born, is capable of independent life; but the 

 size increases up to the normal period of birth, nine months, 

 when it weighs from five to nine pounds, or an average of about 

 seven, varying between the extremes of three and a-quarter up 

 to twelve and even more in some rare instances. 



Foetal life has some remarkable peculiarities, which we may 

 well consider. The formation of germs in the ovaries, their 

 periodical development and expulsion, their impregnation or 

 fecundation by spermatozoa formed in corresponding male 



