That gaseous substances pass through the fetal barrier of tissues was proved by Zweifel, Cohnstein, and Zuntz. Zweifel showed that chloroform administered to the mother rapidly reaches the fetus. As early as 1817, Mayer proved the passage of cyanide of potassium. Since then we have been made certain of the transmission of iodide and ferrocyanide of potassium, salicylic acid, bichloride of mercury, methylene blue, and many other substances. Krönig and Futh, in 1901, determined that the maternal and the fetal blood freeze at the same temperature, which indicates that they possess equal osmotic power, and that osmosis may occur in either direction. Some bacteria do not get through to the fetus, but a few do get in. Tubercle bacilli were found in the fetus by Birch-Hirschfeld[139] in 1891, and Schmorl[140] demonstrated them in 50 per cent. of the placentas in one series of examinations. Bar and Renon[141] found them in the blood of the umbilical cord in two of five cases. Actual congenital tuberculosis is possible, though very exceptional: the bacteria either pass through the wall between mother and fetus, or destroy this wall and then get in. Smallpox, measles, and scarlatina, the causes of which have not yet been demonstrated; typhoid, cholera Asiatica, pneumonia, bubonic plague, erysipelas, pus infection, anthrax, syphilis, febris recurrens, and malaria have already been demonstrated in the fetus. Lynch of Johns Hopkins collected sixteen cases of typhoid in the fetus. I found the typhoid bacillus in the liver and kidneys of a still-born fetus whose mother was ill with typhoid fever; this case was not among those collected by Lynch. [139] _Arbeiten d. pathologisch. Instit. zu Leipsig._ Jena, 1891. [140] _Münchener medizinische Wochenschrift_, 1904, vol. li, p. 1676. [141] _L'Obstétrique_, vol. i, p. 69.