Development of the Mammal 329 



they have probably been formed by the simul- 

 taneous fertilization of two ova, each of which 

 developed into a normal individual, either male 

 or female. Duplicate twins, on the other hand, 

 are usually so much alike as to be with difficulty 

 distinguishable from each other. They may 

 be explained by supposing the ovum to have 

 separated into two parts, at the two-cell or 

 other early stage of development, and each 

 part to have developed into a normal fetus. 

 Such duplicate twins may be produced by cut- 

 ting apart the blastomeres of the egg of some 

 of the lower animals, in the two-cell stage, 

 when each blastomere will develop into a small, 

 but otherwise normal embryo. 



Should the separation of the parts of the 

 human embryo not be complete, or should two 

 embryos develop in too close proximity in 

 the uterus, a double monster may be formed, 

 the extent of fusion varying from a compara- 

 tively slender cord, as in the famous Siamese 

 Twins, to an almost complete fusion, so that 

 one foetus may look like a mere parasite upon 

 the other. 



