THE BIOLOGY OF TWINS 



Light Thrown on Many Interesting Problems by a Study of Twinning How 

 Twins Are Produced — Critical Experiments in Sex-Determination 



TWIN animals represent only the 

 extreme of a process which is 

 found in most living things. In 

 a sense, a man's right hand is the 

 twin of his left hand; or if his body were 

 split in halves from top to bottom, 

 one-half could properly be regarded as 

 the twin of the other. The essence 

 of twinning is therefore "bilateral 

 doubling," which carried nearly to the 

 limit produces Siamese twins, or ex- 

 tended one step farther results in a pair 

 of children who are wholly separated, 

 each one a distinct individual. 



The wide range of twinning makes it 

 interesting to every one, and brings 

 in many far-reaching biological prob- 

 lems. H. H. Newman has brought 

 together many of the most important or 

 striking facts in a little book^ called 

 "The Biology of Twins," in which he 

 considers twins of all kinds in mammals, 

 but particularly the twins of the arma- 

 dillo, which he thinks furnish "the only 

 kev to the mechanics of human twin- 



nmg. 



TWO KINDS OF TWINS 



Professor Newman accepts the cus- 

 tomary division of twins into two classes. 

 In one case (to which the first para- 

 graph above, refers) two individuals are 

 ]3roduced from a single egg. They are, 

 naturally, ahvays of the same sex, and 

 closely resemble each other. They are 

 commonly spoken of as "duplicate" or 

 "identical" twins. In the other case, 

 two individuals are produced from two 

 separate egg-cells, and they are simply 

 ordinary brothers and sisters who hap- 

 pen, so to speak, to develop simul- 

 taneously. Whether they will be of 

 the same sex or of opposite sexes is just 

 about an even chance. • 



The best evidence as to the relative 

 numbers of the two kinds of twins is, 

 in the opinion of Professor Newman, 

 that furnished by the statistics of J. B. 

 Nichols. He found the sex of twins to 

 be distributed as follows, in a large num- 

 ber of cases he collected ; 



Both males 234,497 



Opposite sexes 264,098 



Both females 219,312 



Roughly, this means for the three 

 classes a ratio of 1:1:1. Since in ordi- 

 nary births, the sexes are produced in 

 approximately equal numbers, it is 

 evident that, if all twins were nothing 

 more than ordinary brothers and sis- 

 ters, the ratio ought to be 1:2:1; there 

 would be as many pairs of boys as pairs 

 of girls, and in cases where the twins 

 were of opposite sexes, the number of 

 each sex would be equal. 



From this it is concluded that about 

 one-half of the same-sexed twins really 

 stand for only one individual to the 

 pair; they are identical twins. If this 

 reasoning is sound, about one-fourth of 

 all twins born derive from a single 

 fertilized egg cell; the other three- 

 fourths are not "identical" or "du- 

 plicate" twins, but ordinary fraternal 

 twins. 



HOW TWINS ARE PRODUCED 



The development of ordinary twins is 

 sufficiently explained by saying that 

 they result from the simultaneous fer- 

 tilization and development of two dis- 

 tinct egg-cells. The production of iden- 

 tical twins is more obscure. After an 

 egg-cell is fertilized it splits in two; 

 then each of the halves again divides; 

 and so the division continues almost 

 indefinitelv until the whole bodv is 



1 The Biology of Twins, by Horatio Hackett Newman. vSmall 12mo, pp. 185. The Uni- 

 versity of Chicago Science Series. Price, SI. 25 net, postage extra. Weight 1 lb. University of 

 Chicago Press, 1917. 



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