66 INTRODUCTION TO SEXUAL PHYSIOLOGY 



developed at the posterior end of the embryonic area. These 

 structures, together with the rudimentary allantois, form the 

 umbilical cord, and this becomes covered with embryonic ectoderm 

 by which it is bound to the amnion. The cord constituted in 

 this way connects the embryo with its membranes and so with the 

 parent. It is fully developed about the sixth week of pregnancy. 

 Owing to the growth of the hind part of the embryo beyond the 

 original position of the cord this structure by the time of birth 

 comes to be situated about mid-ventrally. 



Amnion 



Heart Yolk sac 



Body stalk 



Fig. 46. — Human embryo, 2-6 mm. long. 

 (After His, from (t'ra>/s Anatoiaij.) 



As the embryo develops, and its sides close in, some of the 

 space between the somatopleur and the splanchnopleur becomes 

 separated off from the rest of the space and constitutes the coelofn 

 or peritoneal cavity. The further growth of the amnion results in 

 the closing in of the space between it and the chorion. This is the 

 condition at birth when the two membranes are still contiguous. 



The growth of the amnion is accompanied by the formation 

 of a clear fluid, the liquor amnii, which makes its appearance 

 shortly after the formation of the amniotic space. By the 

 second month of pregnancy the amniotic sac is considerably dis- 

 tended by the liquor. This fluid serves as a protective covering 

 for the embryo, keeping it from the effects of jar or outside 

 disturbance, such as injuries to the abdomen of the mother, 



