FROM ONE OR TWO EGG CELLS? 
The most obvious difference between these twins is that one drinks faster than the other. In 
features the resemblance is close. 
Yet no one can say from mere inspection whether they 
represent the twinning that is due to the fertilization of two egg cells, or the twinning that 
is due to a single egg cel! splitting in halves; and as the two kinds of twinning are very 
likely inherited in a different manner, the study of the problem is made difficult. 
graph from the Nursery Studio, Washington, D. C. 
of twins, a decided handicap for the 
student of heredity. 
THE HEREDITARY TENDENCY 
The kind of evidence that one gets 
as to the heredity of twinning may be 
indicated by reference to a group of 
fifty St. Louis families. The investi- 
gator had no knowledge of any of these 
families until in each case, the birth of 
twins was reported to the bureau of 
vital statistics. On looking into the 
family histories, it was learned that 
these fifty new-born pairs of twins had 
171 older brothers and sisters born 
singly and twenty (ten pairs) who were 
twins. The frequency of twins among 
the brothers and sisters of twins then 
is about 1:18. In the mothers’ fra- 
ternities there had been 318 single births 
and ten pairs of twins (1:32), and in the 
fathers’, 219 single and eight pairs of 
1 There is indication that this is also the case with Shropshire sheep. 
Photo- 
(Fig. 4.) 
twins (1:37). Comparing these figures 
with the ‘normal incidence’’ for 
St. Louis (1:90.6) one is justified, 
especially since essentially similar figures 
are obtained from more extensive data, 
in concluding that twin production is 
frequently a family peculiarity. 
Analyzing the individual families, 
evidence is found that what seems to be 
biovular twinning is hereditary in the 
direct female line. The tendency to 
uniovular twinning likewise seems to be 
transmitted through the female and, 
since the incidence of twins is higher 
than normal in the fraternities of the 
fathers of twins,!! it is probable that 
it may also be transmitted through the 
male. Whether there is any relation 
at all between the two types is an open 
question. 
While some of these families furnish 
beautiful charts indicative of an hered- 
See Rietz and Roberts, 
Journal of Agricultural Research, September, 1915. 
201 
