92 CARNEGIE INSTITUTION OF WASHINGTON. 



ovary than in the other, whereas in the same animal the embryos tend 

 to be evenly distributed in the two uterine horns. A tabulation of 

 500 pregnant sows indicates that in about one- third of them there 

 must have occurred a migration of one or more ova across the midline. 

 On the other hand, in sows that had recently ovulated (within three 

 days), and where the ova were en route through the Fallopian tubes, 

 it was found that in at least 96 per cent of the cases there were the 

 same number of ova in the tube as there were discharged follicles on 

 that side, indicating that ova pass directly into the homolateral tube 

 and that external migration to the tube of the opposite side is rare or 

 non-existent. These findings make it clearly evident that internal 

 migration of the fertilized ovum is of common occurrence in the sow. 



Permeability of the Placenta. 



The placenta may be spoken of as a cellular membrane separating 

 the circulation of the mother from that of the fetus, and its permea- 

 bility constitutes one of the main factors in fetal metabolism. The 

 problem of placental permeability is as complicated as it is funda- 

 mental; it extends from the general problem of the permeability of 

 the cell and the resistance of membranes to the question of the specific 

 ability of the placenta as an organ to select, change, and perhaps 

 S3mthesize materials needed for the nourishment of the fetus. As a 

 restricted phase of this problem. Dr. R. S. Cunningham has investi- 

 gated the fluid and salt interchanges between mother and fetus. 

 Selecting cats in the later stages of pregnancy, he injected into the 

 venous system balanced solutions of potassium ferrocyanide and 

 iron ammonium citrate. Owing to the fact that these salts can be 

 precipitated as Prussian blue, they possess the advantage of being 

 easily followed, both in the route traversed and their ultimate location. 

 It was found that the maternal endothelium is easily permeable to 

 both salts, as is also the fetal endothelium. The fetal ectoderm, 

 however, reacts differently to the two salts, in terms of both per- 

 meability and length of time required. In experiments of short 

 duration no trace of either sodium ferrocyanide or iron ammonium 

 citrate was present in the amniotic fluid, fetal urine, or tissue extract. 

 In those of longer duration sodium ferrocyanide was found in the 

 fetal urine and amniotic fluid, but in none of the experiments could the 

 slightest trace of iron ammonium citrate be demonstrated in the fetal 

 tissues. Examined histologically, the placenta in the shorter experi- 

 ments showed Prussian blue in the maternal endothelium, but none 

 within the ectoderm. In the experiments of longer duration blue 

 granules had partially penetrated the ectodermal layer but were not 

 present in the fetal endothelium. In other words, sodium ferro- 

 cyanide penetrates through the entire placenta and is soon detected 

 in the fetal urine, whereas iron ammonium citrate is arrested in the 



