MIRACLES OF SCIENCE 



"identical" twins. It is matter of familiar experience 

 that human twins are occasionally born that are so 

 closely similar in all their physical and mental traits 

 as to be distinguished with difficulty one from the 

 other. Such twins, in the view of the physiologists, 

 have been developed from a single ovum, by some 

 accidental duplication of the laboratory experiment 

 just related. 



These identical twins are always of the same sex, 

 and we may think of them as representing, like Dr. 

 Driesch's fishes, a curiously divided personality. Ac- 

 cording to the normal scheme of things, they repre- 

 sent the hereditary potentialities of a single individ- 

 ual masking in the guise of two physical organisms. 



The same comment obviously applies in even 

 greater degree to the well-known cases in which the 

 bodies of twins are not altogether separated; the 

 best known of such cases being that of the Siamese 

 twins. This condition also has been duplicated in 

 the laboratory, where by retricting the embryo of a 

 sea-urchin without actually bi-secting it, Professor 

 Loeb has developed two adult organisms joined to- 

 gether by a bridge of tissue. 



The constriction of the embryo is effected in a 

 curious way in Professor Loeb's experiment with the 

 sea-urchin. The egg is put into diluted sea water, 

 and the weakened salt solution causes the egg to 

 burst because of its osmotic pressure. Part of the 

 egg content flows out without becoming detached 

 from that which remains within the cell. The rup- 

 tured cell-wall itself serves as the constricting agent, 

 and a dumb-bell shaped embryo is formed. 



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