620 TEXT-BOOK OF EMBRYOLOGY. 



an encircling band of amniotic tissue might cause constriction or even ampu^ 

 tation of some of the extremities, or of some of the digits. Under the same 

 -head there might also be included certain disturbances possibly caused by the 

 umbilical cord. The cord by becoming wound around the neck or extremities 

 and interfering with development may even cause the death of the foetus. 



Causes Underlying the Origin of Monsters. 



Within the past century the old grotesque notions that monsters were the 

 results of supernatural influences or of sexual congress with lower animals have 

 been overthrown as teratology has been placed upon an embryological basis. 

 The very old belief that impressions on the maternal senses may influence the 

 development of the embryo is still held by those who possess little or no scien- 

 tific knowledge, and is not uncommon even among gynecologists and obstetri- 

 cians. While remarkable cases of coincidence have been recorded, there seems 

 to be no proof whatever that maternal impressions are reflected upon the child 

 in the uterus. On the other hand, there has gradually accumulated a large 

 amount of negative evidence obtained from experimental work. The results of 

 this work have been such as to indicate that external influences mechanical or 

 physico-chemical cause the production of monsters. 



Opposed to the theory that monsters are due to external influences is the 

 view that their cause lies within the germ, that is, that some inherent defect in 

 the constitution of one or both of the parental germ cells is brought out in the 

 new organism that develops after their union. According to this theory, 

 therefore, heredity is the important factor in teratogensis. While the oc- 

 currence of defective conditions in the germ cells cannot be demonstrated, the 

 apparent influence of heredity in the production of malformations has long been 

 recognized. Certain malformations, even so great as to put the embryo or 

 fcetus in the class of monsters, have been known to occur in families through 

 successive generations. Such cases may be mere coincidences, yet more prob- 

 ably they are indicative of hereditary influence. 



All the theories, therefore, are concerned with the question "whether the 

 conditions that produce a monster are germinal and hereditary or are external influ- 

 ences acting upon a normal germ" (Mall). Some defend the germinal or 

 hereditary factor as the most potent cause in the production of malformations, 

 while others just as strongly advocate the view that normal or abnormal 

 development depends largely upon external factors. It does not seem possible 

 to deny the importance of heredity in the development of the normal organism; 

 nor, on the other hand, can the importance of external influence, of environ- 

 ment, upon normal development be denied. The same factors may be con- 

 sidered as active in abnormal development, and it does not seem that either 



