Embryology 295 



Other organizers appear during the development of the embryo to 

 control organ differentiation in specific areas. This early one, however, 

 is the primary organizer of the embryo. 



Twins and Twinning. — Much of the above information has throv^n 

 light upon the problem of twinning as it occurs in animals. In those 

 animals normally having multiple births, the young are usually the 

 result of the fertilization of individual ova. In human beings, fraternal 

 twins are of this type and are no more alike than are individuals born 

 of the same parents at different times. They may even be of different 

 sexes. 



The other type of twins, the identical tnnns, arises from a single 

 egg. Naturally being from a single egg, these individuals have identical 

 inheritance and are of the same sex. Experimentally, twins can be 

 produced in the frog by separating the first two blastomeres. Due to 

 the extensive potentiality of these early blastomeres, this is entirely 

 reasonable. In mammals, this method of twin production does not seem 

 quite possible inasmuch as the zona pellucida prevents the isolation of 

 these early blastomeres. The zona pellucida, it will be recalled, is ex- 

 tremely tough and does not disappear until cleavage is well established. 

 In these eggs, it appears that twinning is due to the development of 

 two inner cell masses, each with its own axis of polarity or from a single 

 organization center divided either by fission or budding. In any event, 

 twinning of this type must be due to some change in organization 

 centers. 



