Heredity and sex . pathologists. Thereare hundreds of recorded cases. In all of these bothindividuals are of the same sex, both are malesor both are females. There is good evidence to showthat these double types have come from a single fer-tilized egg. They are united in various degrees (); only those that have a small connecting regionare capable of living. These cases lead directly tothe formation of separate individuals, the so-calledidentical twins. Galton was one of the first, if not the first, to recognizethat there are two kinds of twins — identical twins andordinary or frat
Heredity and sex . pathologists. Thereare hundreds of recorded cases. In all of these bothindividuals are of the same sex, both are malesor both are females. There is good evidence to showthat these double types have come from a single fer-tilized egg. They are united in various degrees (); only those that have a small connecting regionare capable of living. These cases lead directly tothe formation of separate individuals, the so-calledidentical twins. Galton was one of the first, if not the first, to recognizethat there are two kinds of twins — identical twins andordinary or fraternal twins. SPECIAL CASES OF SEX-INHERITANCE 237 Identical twins are, as the name implies, extremelyalike. They are always of the same sex. Thereis every presumption and some collateral evidenceto show that they come from one egg after fer-tilization. On the other hand, amongst ordinarytwins a boy and a girl, or two boys and two girls, occurin the ratio expected, on the basis that their sex is tttffff \ .A. DIAGRAM SHOWING THE INTERRELATIONS OF THE VARIOUS SORTS OF OlPLOPAGI ANODUPLICATE TWINS, ILLXJSTRATIVE OF THE THEORY. ADVANCED IN THIS PAPER. FURTHER EX-PLANATION IN THE TEXT. Fig. 110. — Diagram showing different types of union of double monster (After Wilder.) not determined by a common external or internalcause. Since fraternal twins and identical twins showthese relations at birth and from the fact that theyhave been in both cases subjected to the same condi-tions, it follows with great probability that sex insuch cases is determined before or at the time offertilization. This conclusion finds strong support from the condi- 238 HEREDITY AND SEX tions that have been made out in the first reported that all the young of a singlelitter are of the same sex (Fig. 111). The statementhas been verified by Newman and by Patterson on alarge scale. In addition they have found, first, thatonly one egg leaves the ovary at each gestative period;and second, t
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Keywords: ., boo, bookcentury1900, bookdecade1910, booksubjectsex, bookyear1913
