STRUCTURE AND DEVELOPMENTAL RATE 259 



has been fully considered above. The 'quiescent period' in the 

 armadillo egg is probably the result of lack of oxygen and thus 

 the cause of polyembryony. 



Twinning or polyembryony may be considered a typical method 

 of asexual reproduction, and its occurrence in mammals and 

 other vertebrates makes the phenomenon of so-called 'alterna- 

 tion of generations' universal among animals. 



7. The degree of duplicity in double individuals depends upon 

 the original distance apart of the embryonic buds on the blasto- 

 derm. 



The relative sizes of the two components in double specimens 

 vary widely. In many double individuals the two components 

 are practically equal, while in others one component is of normal 

 size and the other component in a series of specimens varies from 

 slightly below normal size down to a very small mass. This size 

 difference between components is in no way associated with the 

 degree of duplicity. 



8. In double individuals in which the two components are 

 equal in size they are both normal in structure. When the two 

 components of a double specimen are unequal in size, the larger 

 component is almost always normal in structure, and the smaller 

 component is always deformed. The degree of deformity in the 

 smaller component varies directly with the extent of difference 

 in size between the two components. 



9. As the large component reaches adult size the lesser com- 

 ponent may have become so relatively small as to be represented 

 by a nodular mass on the body of the larger, or it may be lost to 

 sight entirely as a twin inclusion. Such conditions make it evi- 

 dent that.doubleness and twinning are actually more frequent 

 than records would indicate. 



10. The types of defects and the degree of deformity exhibited 

 by the smaller component are exactly similar in kind and degree 

 to the deformities found among single individuals. This fact 

 renders the double individual with unequal components a most 

 valuable key to an understanding of the cause of all monstrous 

 development. The two components are from identical germinal 

 origin and are developing in organic connection in exactly the 



