344 



Scientific Proceedings (130) 



the placentalia in terms of the layers of tissue which separate 

 the maternal from the fetal blood-streams; but it must also be 

 noted that the same layer of tissue may be of very different 

 morphology in different species. In view of these differences it 

 seems unfortunate that the vast majority of observations which 

 have been made upon the passage of any substance from mother 

 to fetus, or the reverse have been carried out on single species. 



Among the few observers who have made comparative ex- 

 periments on different species are Romer 2 using tetanus antitoxin, 

 and Wislocki 3 using vital dyes. Romer found that the antitoxin 

 passed readily in the human, occasionally in rodents, and never 

 in sheep or cows. Wislocki found that trypan blue passed in 

 small amounts in the rabbit and guinea-pig, but never in the 

 cat and dog. In his discussion he suggests that both his and 

 Romer' s results may be dependant upon the varying complexity 

 of the placental barrier in the animals studied. 



Experiments have been reported elsewhere 4 in which it was 

 shown that sodium ferrocyanide passed through the placental 

 barrier in the cat from mother to fetus, while iron ammonium 

 citrate was held up by some mechanism located in the fetal ecto- 

 derm. Further experiments have demonstrated that in the rab- 

 bit both the salts passed from mother to fetus, but it was found 

 that the sodium ferrocyanide passed the placental barrier some- 

 what more easily than the iron ammonium citrate; this was 

 shown by the fact that the ferrocyanide appeared in the fetal 

 blood before the citrate, and always remained in greater con- 

 centration. 



The placenta in the rabbit belongs to Grosser's hemo-chorionic 

 type, while that of the cat is endothelio-chronic. But in ad- 

 dition to having an intact maternal endothelium the cat's pla- 

 centa has a much thicker and more complex chorionic ectoder- 

 mal layer than that found in the rabbit. It is possible that with 

 the simplification of the placental barriers there is a decreasing 

 amount of placental control; and a more widespread reduction 

 of the activities of the placenta to the laws of osmosis and dif- 



2 Romer, A., Bcitr. 2. exper. Therap., 1904, H. 9. 



8 Wislocki, G. B., Contrib. to Embryol., No. 62, Carnegie Publ., No. 276, 

 89-101. 



4 Cunningham, R. 8., Amcr. Jour. Phys., 1920, liii, 488-494. 



