890 REPRODUCTION, DEVELOPMENT, GROWTH AND DEATH [CH. LIX. 



them by the uterine arteries and away from them by the uterine veins. 



Into these blood-filled spaces the vascular foetal villi project ; hence 



it is easy for exchanges to 

 take place between the 

 foetal and the maternal 

 blood, though the two 

 blood-streams never mix to- 

 gether. Oxygen and nutri- 

 ment pass from the maternal 

 blood through the coverings 

 of the f cetal vessels into the 

 fcetal blood, and carbonic 

 acid, urea, and other waste 

 products pass in the con- 

 trary direction. The fcetal 

 blood is carried to the 

 placenta by the umbilical 

 arteries, which are the ter- 

 minal branches of the aorta 



* ^ ne ICetiUS ; these paSS to 



ment of membranes and placenta than that shown in the T)lacenta bv the Umbili- 

 fig. 574. 1, Uterine muscle ; 2, placenta ; 3, yolk-sac ; , * -, J . 



4, fused decidua vera and capsularis ; 5, primitive blood- Cal COrd, and the blOOQ IS 



i ,1 -i ,1 i 



returned, through the COrd, 



ty ftiQ umbilical Vein. 



The amniotic fluid consists of water containing small quantities 

 of protein, urea, and salts. It is an exudation from the fcetal and 

 the maternal blood, and the urea in it comes from the fcetal urine 

 which is poured into the amniotic cavity in the later part of 

 pregnancy. Its function is mainly mechanical; it supports the 

 embryo on all sides, and protects it from blows and other injuries to 

 the abdomen of the mother, and from sudden irregular contractions 

 of the abdominal walls. 



Intra-vitam Staining. Within the last few years Goldmann of 

 Freiburg has made observations in which he has injected animals 

 (rats and mice) with certain blue solutions of which pyrrhol-blue may 

 be taken as an example. The animal suffers from no ill effects, the 

 only outward change being that a white rat becomes a blue rat. 

 When the animal is subsequently killed, the stain is found embodied 

 in the granules of specific cells throughout the body. Although it 

 circulates in the blood, no blood corpuscle takes it up, nor has it any 

 effect on the vascular lining. In the skin it is found in the fixed 

 connective tissue cells, but chiefly in free phagocytic cells in the 

 lower layers of the cutis and subcutis. But these migratory cells 

 appear also in every internal organ (except the nervous system), 

 and always in connection with interstitial fibrous tissue; they 



FIG. 576. Diagram representing a later stage of develop- 



vessel of embryo; 6, amnion cavity (outer surface of 

 amnion is fused with inner surface of chorion) ; 7, urn- 

 bilical cord ; 8, foetal villas in placenta. 



