612 THE AMERICAN NATURALIST. [Vor. XXXVI. 
and finally culminates in twins: either incompletely separated, 
as the Siamese twins, or completely separated as in ordinary 
cases. Authorities in the main assign all such cases to a 
“division of the ovum imperfectly effected,” so that “the 
resulting bodies instead of becoming complete twins remain 
united together, frequently having a greater or less extent of 
the body in common”; thus Bateson (94): ‘ Double forma- 
tions arise from a fission of the whole or a part of the original 
embryonic mass." There are two theories as to the process: 
“one holds that they arise by splitting of the original germinal 
area of one embryo, and the other that they arise from the fusion 
more or less of two distinct embryonic areas lying in one ovum ” 
(Hektoen and Riesman, '01, p. 419 e¢ seg.). The present case 
indicates a partial fission of the germinal material, that destined 
to give rise to the anterior parts being wholly divided, while 
some of that destined for some of the hinder organs remains 
undivided. This case comes between the Tocci brothers and 
the Siamese twins. The former of these I have seen, and 
Fig. 9 is from a photograph of them. In these the hinder 
part of the body. is apparently strictly single; there is less 
division than in the calf specimen. The Siamese twins were 
essentially double throughout. In the calf the hind part of 
the body is incompletely double. There are cases very nearly 
like it, of which a brief mention will be interesting. The 
Hungarian sisters, Helen and Judith, had one vertebral column 
as far forward as the second sacral vertebra; there was a 
single anus and rectum; both felt the same desire to defecate; 
there was a single vulva but separate urogenital systems; they 
urinated separately by different urethra; menstruation came 
at different periods in each (Hektoen and Riesman, p. 430). 
In the case of Ritta-Christina the following items are noted 
from the account in Gould and Pyle (97, p. 185): umbilical 
cord and placenta single; heads, necks, arms, and thorax above 
double; abdomen single; lungs imperfectly double, the cen- 
tral lungs being underdeveloped ; one pericardium but two 
hearts; hearts synchronous; digestive organs separate as far 
as lower third of ileum, then single to the anus; stomachs, 
spleen, etc., were right and left; livers right and left were 
