ZOOLOGY 



Identical 

 twins and 

 double 

 monsters 



Poly- 

 embryony 



truded drop of protoplasm had become entirely sepa- 

 rated, both it and the portion left within the membrane 

 developed, producing two individuals from what was a 

 single fertilized egg. When the drop was not com- 

 pletely separated, a double monster was produced, a 

 pair of individuals joined together. 



3. The sort of thing which happened in Loeb's ex- 

 periment occasionally occurs among the higher animals 

 without any intentional disturbance. Calves or chick- 

 ens have two heads or more than the normal number of 

 legs. Even in man we have such cases as that of the 

 famous Siamese twins two individuals connected by 

 a band of living tissue. It is not so generally under- 

 stood that this process of division, carried to comple- 

 tion, gives rise to what are called "identical twins." 

 Such twins, always extraordinarily alike, and of the 

 same sex, are due to the division of a single zygote or 

 fertilized cell. They have exactly the same inheritance, 

 and are thus of great interest to biologists because they 

 afford a means of testing the effects of environment, 

 which is the variable factor, the other being constant. 

 According to the suggested definition of the individual 

 given above, they are parts of the same individual, 

 although of course no one really so considers them. 



4. This division of the fertilized cell not only occurs 

 under experimental conditions and as a rare "accident," 

 but in certain animals is a normal occurrence. There 

 are certain minute insects which regularly exhibit poly- 

 embryony, the zygote splitting up into a considerable 

 number of individuals. Professor H. H. Newman has 

 shown that in the nine-banded armadillo of Texas poly- 

 embryony regularly occurs, four individuals being pro- 

 duced by a fertilized egg cell. These, as in the case of 

 identical twins, are always of the same sex and very 



