INTRODUCTION 3 



No simple definition of twinning can be given, for 

 the term has come to be appHed to at least four distinct 

 situations : 



1. The production from plural eggs (zygotes) of 

 plural offspring in species that habitually produce but 

 a single offspring at a birth. This would include some 

 twins in man, as well as triplets, quadruplets, and larger 

 sets of simultaneously born offspring. Cattle and sheep 

 twins and triplets also belong to this category. 



2. The production of plural offspring either sporadi- 

 cally or as a specific character, from a single fertilized 

 egg (zygote). Such twins, quadruplets, or larger sets 

 of offspring are known as monozygotic, and this mode 

 of reproduction is known as polyemhryony. Two spe- 

 cies of armadillo belonging to the genus Dasypus ex- 

 hibit specific polyembryony; also there appears to be 

 sporadic polyembryony in man and possibly in other 

 species. 



3. The habitual or specific production of paired 

 offspring, each of which is derived from one fertilized 

 egg. Such dizygotic twins are specific for the armadillo 

 Euphractus villosus, and possibly for other species of 

 that genus. 



4. The sporadic production of conjoined twins and 

 various types of double monsters. This is the category 

 to which Siamese twins and similar monstrosities belong; 

 the condition is known to occur in many mammalian 

 species, although very little study of the phenomenon 

 has been made except for man. Some .conjoined twins 

 are evidently monozygotic and are due to processes akin 

 to polyembryony, but others are evidently due to a 

 secondary fusion of dizygotic individuals. 



