DEPARTMENT OF EMBRYOLOGY. 93 



ectodermal laj^er and never reaches the fetal capillaries. There are 

 several reasons to suppose that this selective behavior on the part 

 of the placental ectoderm is based on other factors than osmosis and 

 diffusion, thus suggesting further lines of investigation. 



The problem of placental permeability has been approached in 

 a somewhat different way by Dr. G. B. Wislocki, namxeh^, by the 

 injection of colloidal dyes into the venous system of pregnant cats, 

 guinea-pigs, and rabbits, and into the amniotic cavities of the fetuses, 

 the fate of the dye being subsequently determined by a histological 

 study of the maternal and fetal parts of the various animals. It 

 was found that, when trypan blue or pjTrol blue are injected intra- 

 venously into the pregnant animal, they reach the placenta and are 

 absorbed and stored in the form of granules in the chorionic ectoderm. 

 In the guinea-pig and rabbit, traces of the colloid may pass into the 

 amniotic fluid, as has been described for the mouse and rat, and may 

 even slightly stain the fetus. In the cat, however, these substances 

 are not transmitted, even in traces, to the amniotic fluid or to the 

 fetus. This difference in behavior is explained by Dr. Wislocki on 

 comparative anatomical grounds, differences of architecture making 

 the placenta of the carnivore less permeable than that of the rodent. 

 It was found that vital dyes are also absorbed and concentrated in 

 granules in the cytoplasm of the cells of the vitelline membrane, 

 which in rodents forms the outermost fetal covering. 



'\^'Tien colloidal dyes are injected into the amniotic cavity of the 

 guinea-pig and cat during the second half of pregnancy, they are 

 absorbed in three ways: (1) through the gastro-intestinal tract, (2) 

 through the respiratory tract, and (3) by diffusion through the amni- 

 otic membrane. The fetus becomes vitally stained, but none of the 

 colloidal material passes from the fetal into the maternal circulation. 

 The chief depositories of the dye in the fetus and membranes are the 

 endothelial cells lining the hepatic sinuses, the epithelium of the renal 

 convoluted tubules, the amniotic epithelium, and the endothelial cells 

 of the placental capillaries. The endodermal cells of the yolk-sac 

 are also extremely phagocytic toward vital dj^es. 



The behavior of the placenta toward a particulate substance, such 

 as carbon granules of India ink, injected into the maternal circulation, 

 is quite different from its behavior toward colloids. Dr. Wislocki 

 administered intravenously a filtered solution of india ink into a 

 series of pregnant dogs, cats, rabbits, and guinea-pigs, and found that 

 neither the chorionic epithelium nor the placental endothelium appears 

 to have the power of absorbing or phagocytizing granules as coarse 

 as this. The limit of size of the particles which they are capable of 

 taking up seems to lie somewhere between that of a coarse suspension, 

 such as india ink, and an ultramicroscopic dispersion, such as trypan 

 blue. The placenta and fetal membranes in these experiments 



