THE VERTEBRATES 



451 



In order to provide for the nutrition and respiration of 

 embryo mammals, a complicated connection is made be- 

 tween the blood-system of the embryo and that of the mother. 

 Figure 154 shows a rabbit embryo with its surrounding mem- 

 branes. These are abundantly supplied with blood-vessels 

 connected with the embryo's heart. Figure 155 shows the 

 position of an embryo mammal in a uterus. The darkly 

 shaded area around the embryo represents lining tissue 

 (epithelium) of the uterus, and this tissue receives its blood- 

 supply from the heart and arteries of the mother. The tree- 

 like processes 

 shown in Fig. 

 155 are further 

 outgrowths of 

 the irregular 

 processes shown 

 (in black) on 

 the outer mem- 

 branes of the 

 embryo repre- 

 sented in Fig. 

 154. These tree- 

 like processes re- 

 ceive their blood- 

 supply from the 

 embryo's heart 

 through blood- 

 vessels in the 



FIG. 154. Rabbit embryo of twelve days' develop- 

 ment, with its surrounding membranes, which serve 

 for attachment to the lining of the uterus. (From 

 Marshall) 



umbilical cord attached to the embryo at the umbilicus or 

 navel. As a result of this close attachment of the mem- 

 branes of the embryo and the lining of the uterus, the blood- 

 vessels of the two are near enough to allow osmosis. From 

 the maternal blood-capillaries foods and oxygen osmose into 

 those of the embryo, and excretions of the embryo's cells 

 pass into the maternal blood. Solid bodies, like red blood- 



