916 H. H. NEWMAN AND J. THOMAS PATTERSON 
10. Practically all of the types of atypical variation are found 
in the present collection of foetuses. They are shown to be pre- 
determined, in some cases, with remarkable precision, and in 
other cases, only in so far as their general character and location 
are concerned. 
11. A further statistical study of pairing serves to demonstrate 
the truth of this relation. A mechanistic interpretation of pair- 
ing is offered, involving the idea that the differences in the four 
foetuses of a set may be due to the inexactness of the distribut- 
ing mechanism in cleavage. The paired condition is thought to 
be an illustration of the general law that variability decreases 
with the production of like parts. 
12. Sex is the only character absolutely predetermined, but the 
suggestion is made that the greater variability of males, both in 
the species and within the several sets, may be associated with the 
lack of balance in the chromosome complex. 
13. In reply to the possible objection that the variability of 
the four foetuses of a set is not necessarily an index of the possible 
range of variability of the ovum, it is argued that the variation 
between the right and left sides of the body of a single individual 
is of the same grade as that shown to exist among the quadruplets, 
and hence, from the variational standpoint, the single foetus is 
on the same footing as the quadruplets. 
14. The conditions described are shown to be inexplicable on 
the basis of varying environmental factors acting during gesta- 
tion. 
15. The lack of uniformity in the different sets of foetuses, with 
respect to the limits of hereditary control, leads to the realization 
of the complexity of the problem and to an acknowledgment that 
we are still far from a complete understanding of the factors 
involved. 
