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origin of this monster. He considered that one of the embryos was arrested 
in its development, died and was then surrounded by the abdominal wall of 
the other and was thus carried around for years. 
Procuaska ® believed that in the early stages of development of monsters 
the embryonic areas of one was forced into that of another resulting in 
fusion. Von Bakr,’® Bischorr'! and GEOFFRoY ST. Hinarre !* declared that all 
malformations arose from mechanical disturbances. Differing from others they 
attempted to classify malformations into families, genera and species. BiscHorr 
explained “fetus in fetu” as due to the absorption of one ovum by another, 
Sr. Hırame had described a young shark with a divided head. Von Bakr’s 
observations were made upon several monstrosities. Two were living; one of 
these had two heads at an angle of about 60° to each other, while the other 
was divided to about the middle of the body with the duplicated parts at an 
angle of about 110° to each other. The two chick monstrosities represented 
28—30 hours and 36 hours incubation. From his study of these and of two 
others, one of a double trout and another of a double salmon mentioned in 
his article as described by Jacopı in 1765, he opposes the theory of defective 
growth and also the theory that from an originally simply embryo, a polypoid 
growth could develop. He holds that at first the anlage of the embryo the 
axial portion as well as neural tube (the future spinal cord portion) must 
be cleft. 
JOHANN MUELLER !3 states that embryos of higher animals are devisable 
and regenerative so long as they consist of an homogeneous substance or 
mass which latter contains the power of individual organization equally in 
all parts. Therefore, as long as this stage existed, duplications, etc. could 
occur. LEUCKART showed that besides in vertebrates, malformations are also 
seen in plants and crystals. He, therefore, claimed that monstrosities developed 
by the splitting of the original embryo. In this way a complete series of 
malformations and monstrosities from the first state to a complete double 
could be explained. Bearing in mind the cleavage and voluntary fusion of 
lower animals he looks upon excessive growth, that is supernumerary parts, 
as a tendency towards cleavage. He considers “fetus in fetu” a form of 
budding and that Janus symphyonotus is due to the fact that two embryos 
instead of rising above the plane of the embryonic area more often force 
themselves into the yolk whereupon the back to back position results. Other 
forms result from a cleavage that is ventral or dorsal and complete or in- 
complete. 
ALLEN THompson * who studied a chick monstrosity of 16 to 18 hours 
incubation and a goose of four days development found that occasionally two 
cicatriculae are seen on one yolk, but believed that deception is possible; 
Dareste 1° defended the two cicatriculae idea. MEckEL von Hemspacs "7 believed 
that double monsters were enveloped by the same amnion and that the embryos 
were derived by two fused ova in one Graafian follicle. Barkow '° also believed 
this two ova theory, while D’Atron'!® maintained the monovistic theory. 
VALENTINE,”° an adherent of the cleavage theory, believed that monstrosities 
could be artificially produced. He observed the development of a parasitic 
monster in living pike’s eggs. This writer, the first to give good drawings 
31* 
