Vlll INTRODUCTION. 



common Fowl a change of position occurs, so that the left 

 side of the embryo comes to be laid on the adjacent surface 

 of the yelk. Von Baer had remarked, in very rare in- 

 stances in the bird's egg, more frequently in the ovum of 

 the Pig, the embryo lying with its right side towards 

 the yelk ; and he has ingeniously supposed that the re- 

 version of the viscera is connected with this malposition 

 of the embryo"*. If such be the case, transposition of 

 the viscera may be fairly regarded as a mere variation of 

 the developmental process. Again, the aortic arches are 

 liable to vary somewhat in the manner and period of their 

 obliteration. 



It may be fairly assumed that variation is liable to occur 

 in the developmental process at any stage, and disease is 

 undoubtedly liable to do so. When either occurs in the 

 later stages, there is usually no difficulty in tracing the re- 

 sulting malformation to its proximal cause ; but when the 

 first deviation arises from variation in the process of de- 

 velopment or disease in the earlier stages of development, 

 the resulting malformation is often exceedingly complex, 

 so that it is only by a careful examination of a large series 

 of allied malformations that we can obtain any clue to the 

 nature of the original deviation. 



Whatever the causes of embryonic disease and variation 

 may be, there can be no doubt that these conditions 

 react upon each other. Variation is extremely liable to 

 occur as an accompaniment to disease, probably because 

 new conditions arise ; and, on the other hand, disease fre- 

 quently results from variation. Again, disease passes by 

 insensible degrees into variation, and hence no definite line 

 can be drawn between malformations and varieties on the 

 one hand and diseases on the other. Ordinary varieties, 

 as well as the more marked degrees, of variation may be 

 fairly looked upon as the result of variations in the develop- 

 mental process ; they have therefore been grouped together 



* Allen Thomson, Lond. & Edinb. Med. Journ. 1844. 



