452 



APPLIED BIOLOGY 



cells, cannot pass from the maternal to the embryo's blood; 

 and the blood-cells in the embryonic blood-vessels are 

 formed from certain cells belonging to the embryo. How- 

 ever, the important point is that food, oxygen, and excre- 

 tions osmose between the 

 maternal and embryonic 

 blood-vessels in the mem- 

 branes which attach the 

 embryo to the wall of the 

 uterus. 



The membranes which 

 attach the embryo to the 

 uterus constitute the pla- 

 centa. It normally separates 

 from the uterus after the 

 birth of an embryo, and is 

 then itself discharged by 

 muscular contractions of 

 the uterus. 



The fact that an embryo 

 attaches to the wall of the 

 uterus as above described 

 and is thus enabled to get 

 food explains the large size 

 of many mammals at birth. 

 The eggs of all mammals 

 are transparent cells of mi- 

 croscopic size, and embry- 

 onic growth is due to food 

 supplied by the maternal 

 blood-vessels. 



It should be noted that the entire time of gestation is 

 not occupied with the formation of the embryo's organs. 

 For example, an embryo may develop in two or three months 

 so that it has the form and structure of an adult, but may be 



FIG. 155. Diagram of a mammalian 

 uterus showing attachment of an em- 

 bryo to the lining (black in the fig- 

 ure). The umbilical cord, from the 

 ventral surface of the embryo's abdo- 

 men, extends to the tree-like pro- 

 cesses embedded in the lining of the 

 uterus. The black lines in the um- 

 bilical cord indicate arteries and 

 veins connected with the embryo's 

 heart. The openings of the uterus 

 shown at upper right and left are to 

 the Fallopian tubes leading to the 

 ovaries, while the lower opening is 

 the mouth of the uterus through 

 which the mature embryo is finally 

 expelled by muscular contraction. 

 (From Marshall.) 



