386 



THE NEW INDIVIDUAL 



Part IV 



IDENTICAL TWINS 

 Are products of 



A single 

 sperm 



and 



G 



A single 

 egg 



In an early stage 

 the embryo divides 



The halves go 

 on to become 

 separate 

 individuals 



Usually — but not always — identical 

 twins share the same placenta and 

 fetal sac 



But regardless of how they develop, 

 they carry the same genes and are 

 therefore 



Always of the same sex — two boys 

 or two girls 



FRATERNAL TWINS 



Are products of TWO different eggi 

 fertilized by TWO different sperms 



They have different genes and may 

 develop in different ways, usually— 

 but not always — having separate 

 placentas and separate fetal sacs 



Also, as they are totally different in. 

 dividuals, they may be 



>r two girls 



—Or a 



mixed 



pair 



One 

 boy 



One 



girl 



Fig. 19.20. How twins are produced. (From The New You and Heredity by 

 Amram Scheinfeld. Copyright, 1939, 1950 by Amram Scheinfeld, published by 

 J. B. Lippincott Co.) 



