54 



the'^physiology of twinning 



Although it seems to me that Gemmill's data abnost 

 automatically speak forth the fission theory of twuining, 

 it is perfectly obvious that throughout he has in mind 

 exactly the opposite process. He always speaks of the 

 uniting of paired parts or regions into single elements. 

 All of his types of twins are interpreted as cases of more 



Figs. 29-31. — Typical examples of trout double monsters, arising 

 from the partial fission or dichotomy of a single embryonic axis. Fig. 29, 

 the commonest type of trout twin, exhibiting situs inversus viscerum. 

 Fig. 30, a case in which the fission products are unequal, one being 

 decidedly subnormal. Fig. 31, a rather rare type of minimal twinning, 

 the dichotomy being confined to the most anterior structures, the eyes 

 and the forebrain. A horizontal section through such a type is shown in 

 Fig. 57. (Figs. 29 and 30 after Stockard, Fig. 31 after Gemmill.) 



or less complete fusion of two individuals. He implies 

 that conjoined twins are derived from originally separate 

 individuals that have been brought together by the 

 mechanism of germ-ring closure and have fused more 

 or less completely. This idea naturally leads to a far- 

 reaching conception of the mode of formation of bilater- 



