52 THE PHYSIOLOGY OF TWINNING 



9. Union by caudal extremities alone (not seen by 

 Windle himself, but noted by early writers). 



10. Union by ventral aspects at the site of the 

 attachment of the yolk sac. 



11. Parasites. (All cases in which one individual 

 is distinctly smaller than the other and strongly united 

 to it.) 



Windle's classification, though dealing only with 

 superficial characteristics, emphasized, rightly I believe, 

 the degree of duplicity rather than that of union in 

 all cases of incompletely double individuals. His first 

 seven classes are, in my opinion, true instances of 

 anadidymi or products of incomplete fission of a single 

 bilateral primordium. The remaining four classes repre- 

 sent twins derived from separate embryonic axes, but 

 more or less secondarily fused by external parts. These, 

 according to our theory, are separate twins and do not 

 logically fall into the category of double monsters. 

 Windle has no theory of the origin of these forms to advo- 

 cate, his paper being purely descriptive. 



CLASSIFICATION OF GEMMILL (1891) 



In his early paper, a summary read before the Royal 

 Society of London over thirty years ago, Gemmill gave 

 us a very painstaking classification of a large collection 

 of trout twins and double monsters, based on the study 

 of the internal relations of the connected individuals. 

 It will be noted that he emphasizes union rather than 

 duplicity. His classification is as follows: 



Type I. Union in head region: 



a) The twin brains united at the mesencephalon. 



b) The twin brains united at the medulla oblongata. 



