462 ANATOMY FOR NURSES [Chap. XXI 



The passage of the fertiUzed ovum through the tubes requires 

 about eight days, and during this time many thousands of cells 

 are formed and enclosed in a sac called the amnion. The collection 

 of cells surrounded by the amnion is called an embryo. After 

 entering the uterus the embryo attaches itself to the mucous mem- 

 brane, in the upper portion, usually near the opening of the 

 Fallopian tubes. 



Changes in the uterine lining. — The preparation of the mucous 

 membrane of the uterus for the reception of the impregnated ovum 

 includes changes that are similar to those that precede menstrua- 

 tion. The mucous membrane becomes softer, thicker, and highly 

 congested. In this condition it is known as the decidua vera, and 

 the point to which the ovum becomes attached and whicji later 

 develops into the placenta is called decidua scrotina. 



Intrauterine growth. — During the period of intrauterine life 

 growth takes place rapidly. From the union of the ovum, which is 

 ll^ inch (0.2 mm.) in diameter, and the spermatozoon, which is 

 much smaller, there is developed in two weeks' time an embryo 

 which is about the size of a bean. At the end of four weeks it is 

 the size of a walnut, and at four months it is called a foetus, because 

 it has the appearance of a human being, with well-developed eyes, 

 fingers and toes separated, and the external genitals sufficiently 

 formed to determine the sex. The usual duration of pregnancy is 

 nine lunar or ten calendar months, but at the end of sLx months the 

 foetus is sufficiently developed to live outside the mother's body, 

 but it is fragile and requires a great deal of care. 



For further details on the subject of reproduction the student is 

 referred to standard works on physiology and obstetrics. 



