Section XIV 

 TERATOGENESIS 



EDGAR ZWILLING 



INTRODUCTION 



Previous chapters in this book have demon- 

 strated that normal development depends 

 on a harmonious sequence of closely inter- 

 dependent events. Such development is the 

 end product of the expression of the intrinsic 

 potentialities of cells or groups of cells as 

 conditioned and modified by their relation- 

 ship to each other and to the rest of the 

 embryo. What the student of embryology 

 sees under ordinary circumstances are the 

 visible results of these expressed potencies. 

 Prior to this there are intracellvilar re- 

 arrangements and localization of formative 

 materials. As development progresses masses 

 of tissues migrate from one region of the 

 embryo to another. Having arrived at their 

 destinations they in turn may influence ad- 

 jacent tissues and initiate definite develop- 

 mental tendencies. In this manner a pattern 

 of organ-forming regions is laid out. Within 

 each of these regions, then, we have seen 

 how similar dependencies exist in which 

 later parts require the influence of adjacent 

 regions for their normal formation. Subse- 

 quent to the elaboration of the basic pattern 

 for a structure there is a period which is 

 characterized by cellular differentiation and 

 growth. All of these events presumably are 

 consequences of prior chemical and physical 

 processes. In a situation where such a vast 

 array of orderly interactions must occur in 

 order that a normal structure be formed it 

 is not surprising that deviations from the 

 normal are frequently encountered. Dis- 

 turbances in either the spatial or the tem- 

 poral synchronization of the many develop- 

 mental interrelationships may lead to abnor- 

 mal individuals and structures. Many minor 

 deviations which occur are considered to be 

 within the range of normality. Only the 

 more extreme deviants are regarded as aber- 

 rant. 



Variants from the normal, monstrosities 

 of every sort, have fascinated students of 



biology and medicine from ancient times 

 to the present. Many superstitions have been 

 built around them and they have occasioned 

 much fantasy and specvilation. However, to 

 modern students of embryology these terata 

 (the name given the abnormal individuals) 

 provide further material for the study of 

 development. Not only has the application 

 of principles derived from other experimen- 

 tal procedures been instructive in explaining 

 the nature of terata, but the terata themselves 

 have been utilized in elucidating normal 

 developmental relationships. 



Hypotheses concerning the causation of 

 human terata have been numerous and have 

 included such diverse agencies as celestial 

 influences, gods, devils, hybrids (i.e., human 

 X some other animal), maternal impressions 

 and imagination, mechanical pressure (extra- 

 abdominal, intra-abdominal and amniotic), 

 amniotic and umbilical strangulation, dis- 

 ease, faulty implantation, etc. [For an ex- 

 cellent account of the older history the reader 

 is referred to Ballantyne ('04) and Schwalbe 

 ('06-37).] Recent years have witnessed a 

 revival of interest in these problems in 

 medical circles, partly as the result of an 

 increased attention to inherited anomalies 

 but largely because of the discoveries con- 

 cerning the possible role of viruses, blood 

 factor antagonisms, nutritional factors and 

 radiations in the etiology of congenital mal- 

 formations (for recent bibliography see 

 Gruenwald, '47; Morison, '52). 



Embryological studies must eventually 

 provide the basis for an analysis of the prob- 

 lems in human teratology. Since most in- 

 vestigations of the experimental production 

 of monsters have been undertaken on inver- 

 tebrates and lower vertebrates one of the 

 problems has been to what extent the ob- 

 servations and conclusions derived from these 

 studies may be applicable to humans. In- 

 creasing evidence indicates that no unique 

 principles apply to mammals — that any 

 observed differences result from differences 



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