60 THE EVOLUTION OF MAN 



known to occur in some cases of typhoid fever and, in rare 

 instances, of tuberculosis. On the contrary, it has been demon- 

 strated that the toxins and antitoxins of diphtheria, tetanus, 

 and typhoid fever readily pass from the maternal to the fetal 

 blood stream in the placenta. Furthermore, biologists agree 

 that there is no foundation for the more or less general belief 

 that maternal impressions are responsible for malformations 

 of a child. It can be understood, however, that a general dis- 

 turbance of the fundamental metabolic processes in the mother 

 may result in similar disturbances in the fetus and thus may 

 interfere with the normal developmental processes. 



Birth. It has been seen that the human embryo is a para- 

 site which engrafts itself on the maternal tissues and by the 

 remarkable and highly specialized organ, the placenta, is 

 supplied with nourishment and relieved of its wastes. This 

 connection is normally retained for a period of approximately 

 nine months, at which time the 'host,' for reasons largely un- 

 known, refuses longer to support the 'parasite.' Rhythmic 

 contractions begin in the walls of the uterus. These increase 

 in strength and frequency and finally result in the birth of the 

 child. In separating the placenta and tearing the large ma- 

 ternal vessels of the uterus, without causing an excessive 

 hemorrhage, nature performs a wonderful surgical operation. 

 The moment the child breathes and the lungs expand, the 

 course of the circulation is changed. This change is due to 

 the expansion of the lungs and the consequent large flow of 

 blood to them. As the result of this, the opening between the 

 two auricles (foramen ovale) closes, the course of the circula- 

 tion in the heart is also changed and the blood, which was 

 formerly aerated in the placenta, is now aerated in the lungs. 

 Sometimes this interauricular opening fails to close in the 

 proper manner and this permits a mixture of venous with 

 arterial blood which causes a dark color in the skin and, there- 

 fore, what is commonly known as a 'blue baby.' 



