362 Harris HaAvthome Wilder 



In carrying on the study of these forms from this standpoint it 

 becomes thus a matter of no moment if or at what time a secondary 

 deformation occurs. If we have either a true double monster or one 

 of a symmetrical series less than a normal individual we must assume 

 that any lack of symmetry or other deformation is secondary in nature 

 and that the embryo was not deformed at first. We must try to look 

 through all such deformations, which by the very nature of the case 

 are bound to occur frequently, and endeavor to find what was the essential 

 condition of the original type which Nature attempted to produce, 

 to learn the intention of the germ, if the expression be allowed. To do 

 this it will be well to consider the nature of the causes which may pro- 

 duce secondary deformation of an abnormal embryo. 



Naturally the chief of these in cases where the components are to- 

 gether greater than a single individual, is lack of room, and the common- 

 est result of this disadvantage would naturally be the reduction in size of 

 the less favored component, producing a monster which would be classed 

 under the head of "autosite and parasite," the "parasitic monster" of 

 most authors. In my previous paper I followed the usual custom and 

 sharply distinguished this sort from "true diplopagi," i. e., symmetri- 

 cal ones. This view I at present reject, and, while not quite prepared 

 to accept all cases of parasitic monsters as deformed instances of pri- 

 marily symmetrical ones, I feel sure that the most of them are, and that 

 they difi'er from symmetrical monsters merely in the accidental conditions 

 to which they have individually been subjected during development. 

 The criterion of a true diplopage which should in all cases be insisted 

 on is that of homologous union, that is, that the parts of each component 

 by which they are united to each other should be anatomically the same, 

 a criterion which is indeed difficult of application in cases in which the 

 lesser component is very much reduced, but which is evident in by far 

 the greater number of cases. This would still leave open the possibility 

 of the occurrence of other forms of association, such as that of the second- 

 ary fusion of two blastomeres on a common yolk, in which the points 

 of union would be the chance points at which the two embryos first 

 came in contact with one another, and would not be homologous. This 

 latter form of monster is probably common among the Sauropsida, 

 since the occurrence of such fusion in the early stages has been fre- 

 quently observed, although it is not likely that these cases would be able 

 to develop far. A similar secondary fusion of two geometrically im- 

 related embryos might account for cases of included fetus (fetus in 



