485 

factor was pressure upon the embryonic aiea from above. The cleavage must 
be consummated before the complete formation of the ectoderm and entoderm, 
mesoderm and entoderm. The forces of separation seem to work earliest at 
the cephalic extremity of the embryonic anlage and seems to be due to the 
vitelline membrane. If the latter is too tense or the yolk abnormally large, 
then a rupture of the embryonic area must result. The impetus is external. 
RINDFLEISCH ?® placed monsters with supernumerary parts and simple axis 
and double monsters in the group of organopoetic tumor formations and agreed 
with MEckEL that from the fetus, neoplasms may develop in which one finds 
a chaotic new formation of all tissues. Bruch * described a monster Pelobates 
fuscus. At about one fifth of its length distant from the tail the notochord 
was divided and at about 5 mm from the end one division was again cleft 
giving a tridate appearance. He believed that duplication depends upon the 
divisibility of the animal organism. It may occur at any time in the lower 
forms of animal life, but in higher forms it can only occur at a certain period 
in their development, that is from the moment of fertilisation to formation 
of the rudimentary body. 
VircHow *! from his study of the Siamese twins and a two-headed 
nightingale, believed in. the segmentation theory. He agreed with GEGEN- 
BauR* in his observations on a double limax. LEREBOULLET ** in his study 
on experimental fertilisation in pike’s eggs in 1852—1855, made some excep- 
tional observations. He formed several categories or series, of which the 
following five only are of importance: — 
Series 1. Double fishes with two bodies or two heads. In this series 
embryos with two complete separate bodies were never seen. Nor were eggs 
with two yolks or two embryonic areas found. In most instances the heads 
lay against each other, while the bodies were at an angle to each other. 
Fusion can occur only in the earliest stages of development. The cells at 
the point of contact must be elementary organic cells in order to make the 
intergrowth possible. When the heart appears, fusion ceases. 
Series 2. Double monsters with incomplete development of either embryo ; 
eyes, internal ears or heart are not formed, but the notochord is present. 
One embryo may be reduced to a mere nodule with a pulsating heart. The 
nodule may be entirely resorbed. 
Series 3. The embryos are originally individual, but may fuse completely. 
The two heads may lose their adjacent sides completely while the outer halves 
form a normal head. 
Series 4. Triplication. Of this there was but one instance but only two 
hearts were present. 
Series 5. Here this author placed a monster that consisted of one head 
and one tail, but of two intervening bodies. On each side of the median 
line was to be seen a nerve strand, a notochord and a single row of somites 
which were united caudad and cephalad. 
All of the above series he claimed to be modifications of marginal tumors 
(Randwulstes) and the above monstrosities led him to adopt the fusion theory. 
He considered that such tumors may develop into an embryonic body. 
