Chap. 19 DEVELOPMENT 383 



As soon as the villi are well developed, they secrete a hormone (chorionic 

 gonadotrophin) promptly circulated by the blood and easily extracted from 

 the urine. Experiments have shown that human urine of pregnancy has a 

 stimulating effect upon the ovaries when injected into the bodies of immature 

 rats and mice, the basis of the Aschheim-Zondek pregnancy test. In the 

 Friedman test for early pregnancy, the urine is injected into the ear vein of a 

 rabbit. If the woman is pregnant, eggs will be shed from the ovaries into 

 the oviducts of the rabbit in about 24 to 48 hours. Obviously this requires an 

 operation on the rabbit. Physicians most commonly use the much simpler 

 test on frogs. Some of the urine to be tested is injected into a dorsal lymph 

 sac of an adult male frog (Fig. 32.20), usually the common Rana pipiens. 

 The frog is placed in a dry jar for two hours. Some of its urine is then col- 

 lected and examined with a microscope. If it contains sperm cells the preg- 

 nancy is regarded as certain. 



I'terine muscle 

 Remains of volk sac 



Fetal villi of 

 chorion 



Maternal sinus 

 {Intervillous space 



Vecidua basalis 

 Placental septum 



Marginal sinus 



Inised decidua parietalis 

 and capsularis 



Chorion 



A mnion 



Fig. 19.18. Diagrammatic section through the uterus; infant just before birth in 

 the usual position. As in the majority of mammals the yolk sac is present only as 

 an inheritance from vertebrate ancestors. The placenta and other sacs are forced 

 from the uterus as the afterbirth. (Courtesy, Arey: Developmental Anatomy, ed. 

 5. Philadelphia, W. B. Saunders Co., 1946.) 



