DEVELOPMENT FROM THREE TO FOUR WEEKS 277 



contracts German measles during very early pregnancy the fetus will develop 

 heart defects and cataracts of the eye and it may also be microcephalic. It is 

 assumed that even a few red blood cells, or parts of red blood cells, pass 

 through the placental membrane. 



Certainly small, undissociated molecules, such as ammonia, urea, and 

 uric acid pass through freely, since they are found in equal concentration in 

 the maternal and fetal blood streams. Glucose penetrates freely and is stored 

 as glycogen in the placenta and in the fetal liver. Some studies on mammals 

 other than the human suggest that the energy for development comes mainly 

 from the oxidation of carbohydrates. 



The greatest demand of the growing fetus is for amino acids, and these 

 substances are reported to be present in the fetal blood in even higher con- 

 centration than in the maternal blood. If this is indeed a fact, then some work 

 must be done by the cells of the placenta to produce this concentration of 

 amino acids. 



Carbon dioxide and oxygen pass across the placenta freely. The chemical 

 conditions in the fetal blood are such that the hemoglobin takes up more oxy- 

 gen than the maternal hemoglobin at the same oxygen tension. This situation 

 is a result of a lowered oxygen-carrying capacity of the blood of pregnant 

 women as compared with that of nonpregnant women. 



The placenta must show some differential permeability; otherwise the 

 large amount of estrogens in the maternal circulation would influence the sex 

 of the developing embryo. Indeed, if testosterone and estrone are injected 

 into pregnant rats, the female and the male fetus are modified in a male and 

 female direction respectively. Probably some estrone gets through in humans, 

 but not enough to override hormones produced by the fetus. In this connec- 

 tion the suprarenal cortex of the human fetus contains a zone of cells that 

 are believed to secrete an androgen. This androgen may possibly neutralize 

 any estrogen which is able to penetrate the placenta from the mother. 



Development from three 

 to four weeks 



Development from 21 to 26 days is shown in Figure 185. The embryo now 

 possesses about 25 pairs of somites, the arm and leg buds are visible, and 

 a distinct tail bud is present. Its structure resembles that of a chick embryo of 

 about three days' incubation. Three pairs of visceral grooves have developed, 



