The Development of a Double-Headed Vertebrate. 

 By Samuel F. Clarke, Ph.D. 



1 HE mode of origin and development of duplex monstrosities in the vertebrates is one of 

 those interesting questions so beset with difficulties that it is seldom we can gain any 

 direct observations with which to test the existing theories. Any one who examines 

 a considerable number of these double forms cannot fail to notice the varying degrees to 

 which the duplicity is carried. It is generally agreed that two beings who are in the 

 slightest way connected by a band of flesh represent one extreme of the series of double 

 monsters, — as, for example, the Siamese twins, — while the other extreme consists of a 

 nearly normal form in which there are but the slightest indications of duplicity. Between 

 these extremes one finds all degrees of variations from the normal. One of the most 

 generally accepted methods of explaining the origin of these monstrosities is by supposing 

 that two eggs are fertilized and developed at the same time. This theory would account 

 very well for ordinary twins, or for such as the celebrated Siamese twins, where the 

 physical band is slight ; but for other forms, in which the duplicity is but partly expressed, 

 this theory does not answer so well. According to this view, the origin of such a form 

 as is represented in plate 1, fig. 5, would be explained as follows : Two eggs fertilized at 

 the same time had been thrown together, remained united, and developed in such a way 

 as to form the two-headed monster represented. Now this satisfactorily accounts for the 

 anterior portion only, — the two heads. Why there should not be also two bodies and 

 four pairs of limbs as well as two heads is explained (?) by saying that the two ova 

 became united in such a way as to prevent the development of more than the ordinary 

 arrangement posterior to the head. How is it, then, that the part of the animal which is 

 single is so regularly and symmetrically developed ? This would seem highly improbable 

 if two eggs at a very early stage had been thrown together. Each egg, of course, has the 

 tendency to develop one symmetrical organism, like that from which it came. Now, 

 when two eggs become united, it does not seem very reasonable to suppose that these 

 tendencies will remain intact in certain parts of the united mass, so as to produce two 

 heads or two tails, etc., while in other parts we find a perfectly normal development. 

 To accoimt for this, we must suppose that either a part of the mass of one egg loses its 

 tendency to develop and takes no active part in the progressive changes, being merely 

 absorbed by the protoplasm of the other egg, which has retained its tendency or power to 

 develop, or that both portions have become intermingled, and in this enlarged mass there 



