No. 1.] ORIGIN OF HUMAN MONSTERS. 19 
or “Siamese” twins were formed. Frequently they became 
separated and independent animals developed. It often hap- 
pened that the outflow of protoplasm was multiple, and then 
the three, or even more, protoplasmic drops which were 
formed developed respectively into triple or quadruple 
monsters. 
These important discoveries were soon extended to the 
vertebrates by E. B. Wilson,’® who experimented upon the 
eggs of Amphioxus, and by O. Schultze,*? who experimented 
upon those of the frog. Wilson partly separated the blasto- 
meres of Amphionus in the two-celled stage and produced a 
variety of double monsters which developed until the first 
gill-slits were formed. In the gastrula stage almost every 
possible transition occurred between forms slightly expanded 
laterally to those in which the two bodies were joined only by 
a slender bridge of tissue. Incomplete separation of the blas- 
tomeres in the four-celled stage gave rise sometimes to double 
embryos of equal size, triple embryos, one being as large as 
the other two, or rarely to quadruple monsters. Wilson’s 
studies prove, he believes, “that the unity of the normal 
embryo is not caused by a mere juxtaposition of the cells, but 
they indicate that this unity is not mechanical but physiolog- 
ical, and point toward the conclusion that there must be a 
structural continuity from cell to cell that is a medium of co- 
ordination, and that is broken by the mechanical displacement 
of the blastomeres.” 
Oskar Schultze produced monsters in frogs by fixing the 
eggs between two glass plates, and after they had developed 
to the morula stage the plates were inverted. A number of 
the eggs righted themselves, but others grew into double 
embryos. Wetzel'* extended the observations of Schultze 
and showed that there was a flow of protoplasm in each of 
the blastomeres into their upper hemispheres, which may 
*Wilson, Jour. of Morph., 1893. 
%O. Schultze, Verhandl. d. anat. Gesellsch., 1894, and Roux’s Archiv, 
I, 1895. 
*Wetzel, Arch. f. mik. Anat., XLVI, 1895. 
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