THE PRESENT EVOLUTION OF MAN — PHYSICAL 247 



and spermatozoon probably prevented the entrance of 

 the pathogenic microbes, which, judging from the 

 observed behaviour of the micro-organisms of many- 

 other diseases, probably find their nutriment normally 

 in the fluids of the host, and are taken up by certain 

 cells, the phagocytes, only when an attempt is made to 

 destroy them. The escape of the embryo is more easy 

 to understand. As has long been known, there is no 

 connection between the blood-vessels of the mother and 

 those of the foetus, and therefore no blood flows from 

 the one set of vessels into the other. They are, how- 

 ever, in intimate apposition, and are so thin-walled 

 that, while no solids can pass from the parent to the 

 embryo, or vice versd, fluids with absorbed gases pass 

 readily by osmosis. Like other solids, the microbes of 

 disease are stayed in the placenta, — Klein mentions 

 that he has examined the placenta of a guinea-pig dead 

 of anthrax, and found the microbes therein strictly 

 limited to the maternal part of it, — and cannot, there- 

 fore, under normal circumstances infect a healthy 

 embryo from a diseased mother, nor vice versd. Under 

 abnormal circumstances, which may be induced by the 

 action of the micro-organisms themselves, a breach of 

 continuity in the walls of one or more vessels occurs, 

 and the micro-organisms pass from the diseased to the 

 healthy individual. In cases when this does not hap- 

 pen, when a healthy child is born to a diseased mother, 

 or when a healthy mother bears an infected child to a 

 diseased father, it is highly significant that the healthy 

 individual is henceforward, or for a considerable time, 

 immune to the disease ; or at least a healthy mother 

 who has borne an infected child is immune to the dis- 

 ease, as is proved by the fact that she does not contract 

 it when giving suck to her infant, whereas other healthy 

 women become infected if they similarly tend it ; and 

 it may safely be prophesied that the healthy child of 



