1 1 30 REPRODUCTION 



of the exchange is essentially different from that of the more familial 

 forms of respiration. Diffusion of the gases from places of higher 

 to places of lower tension unquestionably plays an important 

 part. But this does not exclude the possibility of a more active 

 process of some other kind, although there is at present no direct 

 evidence of such a gaseous secretion as has been previously discussed 

 in connection with pulmonary respiration (p. 262). The presence of 

 oxydases in the placenta does not throw any light on the question. 

 For there is no proof that they act in transferring oxygen from the 

 one circulation to the other, and oxydases are found in the most 

 diverse tissues. Their significance for the combustion processes of 

 the body has already been alluded to (p. 271 ). 



Salts soluble in water, including not only those necessary for 

 nutrition, like sodium chloride, but many foreign salts, pass readily 

 from the placenta to the foetus, and in general more easily the lower 

 their molecular weight. Such salts as potassium iodide, e.g., when 

 injected into the maternal circulation, appear in the foetus in a very 

 short time. On the other hand, colloidal solutions e.g., of silver 

 or silicic acid do not pass over at all. It is of practical importance 

 that substances like chloroform, ether, and other narcotics, and alka- 

 loids like morphine and scopolamine, when administered in obstetrical 

 practice, may find their way from the mother to the child, although 

 more slowly and more capriciously than the salts. While diffusion 

 and osmosis assuredly take part in the passage of materials from the 

 placenta to the foetus, there is no more reason to conclude that the 

 whole exchange, even for the salts, depends upon such simple physical 

 processes than there is in the case of the exchange between any one 

 of the maternal tissues and the maternal blood. The essential 

 similarity of placental and intestinal absorption, to take one instance, 

 is seen in the mechanism by which the foetus gains the iron required 

 for the development of its haemoglobin. The haemoglobin of the 

 mother appears to be the most important source of this iron. 

 Erythrocytes in all stages of decomposition can be found in con- 

 tact with the chorionic villi, and even in the epithelium covering 

 the villi. These corpuscles come partly from extravasations in 

 the maternal portion of the placenta, but it is possible that the 

 villi also possess the power of haemolyzing intact corpuscles in the 

 circulating placental blood. Iron can be demonstrated by micro- 

 chemical reactions in the epithelial cells of the chorionic villi as fine 

 granules, which increase in size towards the base of the cells. As we 

 pass deeper into the villus towards its central bloodvessel, the 

 granules again diminish in size. The picture is very like that seen 

 in the absorption of iron from the intestine. And if the micro- 

 chemical picture is practically the same, the process by which the 

 iron is absorbed is not likely to be fundamentally different m the 

 two cases (p. 447). 



