material and oxygen passed from the mother to the fetus, and the 

 effete products of fetal metabolism from fetus to mother; it was 

 considered to be a kind of fine sieve, through which percolation 

 took place, or a diffusion membrane that favored osmosis. It is 

 now almost certain that the transmission of substances between 

 the maternal and fetal blood is not merely a matter of physics. The 

 chorionic epithelium is believed by many to be a highly differen- 

 tiated tissue, capable of carrying on complex vital processes, pos- 

 sessing powers of selection, elaboration, and even digestion. Mar- 

 chand has suggested that the syncytium is the chief factor in the 

 absorption of nutritive material from the maternal blood, the Lang- 

 hans layer being more concerned with the transmission of waste 

 products from ovum to maternal blood. Cavazzani and Levi state 

 that there is no correspondence between the quantity of urea in 

 the maternal and fetal blood, that there is more glucose in the 

 former than in the latter, and that the density of the fetal blood 

 is greater than that of the maternal blood. It appears that there 

 are considerable variations in the transmission of substances through 

 the placenta at different periods of pregnancy. Thus, in the last 

 three months, there is a great increase in the iron, potash, and lime 

 stored up in the fetus. In the early months there is a great pre- 

 dominance of soda over potash. 



Various materials may be stored in the placenta. Thus, it un- 

 doubtedly fixes glycogen. It is thought that albuminoid material 

 is transmitted as soluble peptones, though this is not definitely 

 known. There has been some question as to the possibility of the 

 passage of maternal leukocytes through the walls of the villi enter- 

 ing the fetal circulation. Varaldo states that there are more leu- 

 kocytes in the umbilical vein than in the umbilical arteries, there 

 being, on the average, considerably more per cm. in the former 

 than in the latter, and that more of them contain iodophilic granules 

 in the former than in the latter. It has, therefore, been concluded 

 by several that leukocytes normally carry substances (possibly 

 nutriment) to the fetal tissues. This has not been proved, how- 

 ever. In maternal leukocythemia there is no corresponding in- 

 crease in white corpuscles in the fetal blood. 



The placenta acts as a protective barrier against the invasion 

 of the fetus by various poisons. It is more efficacious against some 

 than against others. Porak's experiments on the guinea-pig, for 

 example, show that in this animal copper passes easily, arsenic 

 with difficulty, and mercury, not at all, the poisons being stored 

 to a greater or less extent in the placental tissue. With regard to 

 microorganisms and their toxins, little is known. Many microbes 

 are able to pass from mother to fetus, but nothing is known as to 

 the conditions associated with the transit. It does not appear that 



