ISO THE BIOLOGY OF TWINS 



great as to have a pronounced effect on size and stage 

 of development of different individuals of a set, have no 

 effect on the inheritance of doubling. There are 

 several quadruplets in which the various individuals are 

 pronouncedly different in size, but are practically 

 identical in the character and incidence of doubling. 

 The differentiating factor, therefore, must be within 

 the embryo itself. It seems logical to look to the 

 cleavage mechanism as the probable seat of the irregu- 

 lar distribution to different areas of the blastoderm of 

 the factors of doubling. Presumably, with an ideally 

 accurate cleavage mechanism there would result an 

 exactly identical incidence of doubling in all four fetuses 

 of a given set. That identity in doubling is not realized 

 argues strongly for unequal distribution (or somatic 

 segregation) of factors during cleavage. Moreover, since 

 doubling is evidently as strongly inherited from father 

 as from mother, it seems probable that nuclear elements 

 are chiefly involved, for the cytoplasm of the sperm cell 

 is so small in amount as to be neghgible. 



If we assume that the beginning at least of segrega- 

 tion occurs in the. first and second cleavages, we may 

 suppose that when the factor goes to the first two 

 blastomeres it is pretty certain to appear in both pairs 

 of fetuses; when it goes to each blastomere of the four- 

 cell stage it is Hkely to appear in all four fetuses. If, 

 however, the distribution of .the factor is such that it 

 goes entirely to one of the first two blastomeres and not 

 to the other, we should expect doubling to appear in 

 only half of the fetuses; if, again, only one of the four 

 cells gets the factor, we should have the factor in only 

 one quadrant of the blastocyst and hence should prob- 



