CHAPTER XX. 



TERATOGENESIS. 



MALFORMATIONS INVOLVING MORE THAN ONE INDIVIDUAL. 

 Classification, Description, Origin. 



To give a complete list of the numerous malformations in man, even of 

 those which affect the external form of the body, is obviously beyond the scope 

 of this book. In this chapter it is the purpose of the writers merely to describe 

 in a general way the most striking malformations and discuss briefly the causes 

 underlying the origin of malformations. For further details concerning the 

 subject the student's attention is directed to " References for Further Study " 

 (p. 624). 



The classification of malformations is attended by great difficulties. This 

 is due mainly to the fact that their complexities are not wholly understood. It 

 is due also in a measure to the fact that, apart from certain malformations 

 which always occur in like manner in different individuals, there are so many 

 irregularities and deviations from any type that might arbitrarily be chosen. 

 The classification made by St. Hilaire three-quarters of a century ago, 

 although apparently complete, showed many incongruities as teratology 

 became more firmly established upon an embryological basis. Later 

 Bischoff formulated a division based upon the embryological significance 

 of malformations. This in turn was elaborated by Forster, and Forster's 

 scheme has been adopted by Marchand and others. As knowledge concern- 

 ing teratogenesis is added to, it may be that further changes in classification will 

 be necessary, especially in view of the fact that much light is being thrown by 

 experimental methods upon the origin of malformations. 



Marchand's scheme of duplicate monsters is given here as a convenient one 

 for a comprehensive view of anomalous conditions affecting two individuals. 

 I. Both bodies derived from anlagen which developed from one ovum 

 and which were originally similar and symmetrical: Symmetrical 

 duplicity. 

 A. Both bodies originally complete : Complete duplicity. 



i. The two bodies remain separate; union confined to chorion* 

 Twins; free duplicities. 



a. Both bodies formed alike, each capable of living: Equal 

 monochorionic twins. 

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