392 Duplicate Twins and Double Monsters 



thus primarily, not fonr but two cases, corresponding to the two types of 

 twins, Fraternal and Duplicate. The close connection of IV and III 

 suggests what may have already occurred to the reader, that many cases 

 of compound monsters come under the same category as separate dupli- 

 cates. This is quite probable, but such forms, arising from a secondary 

 fusion, would be asymmetrical and more or less unequal, and would come 

 under the class of autosite and parasite rather than that of symmetrical, 

 or genuine double, monsters.] 



Definitions of Duplicate and Fraternal Twins. — These consid- 

 erations, together with the distinctions made at the beginning of the 

 article, will enable us to formulate distinctive definitions of the two forms 

 of twins, as follows : 



I. Fraternal Twins. — Either of the same or opposite sex and bearing 

 no closer physical resemblance than is usual in children of the same fam- 

 ily. These probably originate as two separate eggs, and any intimacy 

 of association during intra-uterine life (which is never as close as in 

 duplicates) may be attributed to the crowding within narrow limits to 

 which they are necessarily subjected and for which no adequate provision 

 is made such as occurs in mammals in which multiple births are the rule 

 and not the exception. 



II. Duplicate Twins. — Invariably of the same sex and exact or approxi- 

 mately exact physical equivalents of one another, especially in youth, 

 before the modifying influences of environment and habit have had much 

 opportunity to affect them. During intra-uterine life these are more 

 intimately associated than are other twins, and in rare cases this associa- 

 tion is of so close a character as to result in the production of compound 

 monsters. All such cases, whether separate or united, may be referred 

 to one and the same cause, that of some division in the fertilized egg, 

 presumably that of the first cleavage nucleus, in such a fashion as to result 

 in the formation and development of two embryonal areas upon a single 

 blastodermic vesicle. 



Triplets and Other Multiple Births.; — The subject of twins and 

 their intra-uterine relations is not complete without reference to the 

 similar phenomena presented by triplets and the rarer cases of higher 

 numbers at a single birth. According to the statistics of Veit the review 

 of thirteen million birth records in Prussia shows that cases of twins 

 occur once in every 88 births, triplets once in 7910, and quadruplets once 

 in 371,126, and ISTorris, 96, states that twin births occur in New York 

 and Philadelphia in the proportion of 1 to 120, while in Bohemia the 

 proportion is 1 to 60. Mirabeau, 94, states that triplets are most com- 

 mon in multiparous women between thirty and thirty-four years of age. 



