Harris Hawthorne Wilder 419 



as Laloo and Louise L., a comparison of palms and soles or of both, 

 as applied in Part II to the case of separate twins, would be quite 

 possible. 



Separate duplicate twins, thus, have successfully avoided two dangers, 

 (l)that of an incomplete separation of the two hlastomeres of the two- 

 celled stage, and (2) that of a secondary fusion of the later hlastomeres 

 owing to close proximity; in the one case they would have become diplo- 

 pagi, in the other, one tvould have teen an autosite, the other a parasite. 



OTHER RECENT THEORIES CONCERNING THE GENESIS OF 

 COMPOSITE MONSTERS. 



The main principles along which theories on this subject, both ancient 

 and modern, have developed, are the following: (1) the principle of 

 excess and defect, (3) the principle of fusion, (3) the principle of 

 fission, and (-l) the principle of two original fetuses. These principles 

 are for the most part extremely old and in their modern form differ 

 from the ancient mainly in the more definite application made possible 

 by the advance of knowledge of the subject. Thus where Empedocles 

 spoke of an excess of semen, and Democritus of two " seeds " we now 

 speak definitely of polyspermy; the theory of Aristotle, who sought 

 for the origin of such monsters in an excess of material, was the precursor 

 of the theory which postulates a doubling of the determinants in the 

 germ-plasm. Another characteristic of modern theories is that they often 

 make use of two or more of the above principles in combination, although 

 the supporters of the various principles seldom, if ever, agree upon the 

 method of combining them, or in the precise method of their application. 



Since almost every teratologist who describes a case develops a theory 

 concerning its origin, it is manifestly impossible here to attempt a digest 

 of all the recent work in this line, but at the risk of unwittingly omitting 

 many valuable scientific contributions to the subject I will present here 

 what seem to be the principal theories of the present time, selecting as 

 their exponents those authors who have treated the subject in a general 

 way and who have familiarized themselves with a large number of cases. 



I. — The first of the modern theories to be considered postulates an" 

 EARLY TOTAL Fissiox OF THE EMBRYO^ followed in the case of compound 

 monsters by a secondary fusion of the two parts. Of this school the 

 pioneer seems to have been Fisher, 66, who, although writing nearly 

 forty years ago, anticipated many of our most advanced ideas on the 

 subject. He says that double monsters " are invariably the product of a 

 single ovum, with a single vitellus and vitelline membrane, upon which 

 a double cicatricula, or two primitive traces, are developed. The several 



