ORIGIN OF MONSTERS 559 



of some toxic products of pathologic metabolism. The mon- 

 strosities thus produced resemble very much those occurring 

 spontaneously in man and in other mammals. In other words, 

 effects of unknown processes occurring in nature have been practi- 

 cally duplicated on a very large scale by the laboratory experiment. 



In the latter the initial cause is known, because controlled by 

 the experimenter. Some further steps in the sequence of events 

 can, as I have recently convinced myself, partly be observed, 

 and partly deduced from anatomical examinations of great num- 

 bers of monsters. The results of the latter agree well with those 

 of anatomical examinations of the spontaneous monsters of 

 mammals. Provided that in nature the same or very similar 

 causes are responsible for the origin of monsters, the whole 

 problem of teratogenesis m nature is — theoretically at least — 

 within the control of the experimenter. 



On the other hand, the experimental results recorded by pre- 

 vious work in embryology and teratology all seem to point to 

 the conclusion that of teratogenic factors there are many, indeed 

 almost as many probably as there may be agents injurious to 

 organic existence. On what basis then, may it properly be 

 asked, can experiments be performed with the aim of control 

 of the causes, if the latter ones can only be a few out of a very 

 great niunber of possible ones? 



The problem, while ob\dously beset with many difficulties, is, 

 however, not quite as elusive as it would at first appear. For, 

 while there is probably an endless number of factors which, if 

 acting on the egg, might cause it to develop in an atypical 

 manner, it is, for our purposes, necessary to consider only those 

 which can reasonably be conceived as acting in the mammafian 

 body under certain unusual conditions. The latter are, as we 

 shall presently see, relatively few. 



These factors must be either of a physical or of a chemical 

 nature. 



Of the physical factors which might interfere with the t>T3ical 

 development in utero of mammals only pressure and an increase 

 in temperature could be imagined. The former has, indeed, for 

 a long time been regarded as the cause underlying the origin of 

 monsters. The inadequacy of this mechanical theory of terato- 



