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Double Monstrosities. 
VIII. THE ORIGIN AND PRODUCTION OF 
DOUBLE MONSTROSITIES. 
HE problem as to the origin of such monstrosities has 
been formerly attacked in a somewhat a priori manner 
by the aid of two antagonistic theories of fission and 
of coalescence. Recent advances in the study of normal 
development, as well as experiments on the artificial pro- 
duction of monstrosities by pressure, and the unsymmetrical 
application of heat, have shown that the question was 
formerly inaccurately put. Dr. L. Gerlach has recently 
thrown a new light on this subject in a treatise on the origin 
of twin monstrosities among the higher vertebrates. He 
shows that such a phenomenon as the coalescence of two 
mature ova does not occur, and that the malformation 
extends back to the earliest time of the development of the 
embryo. Hence the question to be put is : do we first 
perceive, in double monstrosities, the initial stages of a 
single individual or of two ? 
The existence of two area pellucidce and of two separate 
vitelline membranes upon a single yolk may be distinctly 
observed in birds, but not in mammals. In these cases 
there may be, if not a coalescence of two preformed embryos, 
at any rate a confluence during their formation. Where 
there is only one area pellucida, there may be developed 
within it one primary stripe or two ; if two, they may either 
be developed distinctly from each other, thus producing 
twins, or they may interfere with each other, and thus 
eventuate in double monstrosities. Even a single primitive 
stripe may become bifurcated, and thus produce a double 
monstrosity, in which the anterior extremity is doubled, but 
the posterior single. 
The origin of such anteriorly doubled monstrosities was 
examined by the author in two decided and three approximate 
cases. The surface of a fresh hen’s egg was coated over 
with varnish, except a V-shaped part, at an equal distance 
from the blunt and the pointed end at right angles to a line 
connecting both. The access of air was thus limited to this 
spot. The egg was let remain in such a position that the 
free spot remained turned upwards. Thus probably the 
incipient primary stripe lies in the longitudinal direction of 
the V. The author was led to this process by the con- 
