No. 1.] ORIGIN OF HUMAN MONSTERS. 17 
physical conditions may influence and modify normal develop- 
ment and thereby produce a variety of merosomatous terata. 
During the following seventy-five years a great amount of 
experimental work was done upon chicks by numerous inves- 
tigators, which showed that the varieties of monsters pro- 
duced were quite constant, no matter what agent is used, but 
no single variety could be produced with certainty. It was 
found impossible to experiment with precision, for a certain 
per cent of eggs would produce one or more varieties of 
deformed embryos. 
EXPERIMENTAL. 
RECENT WorK UPON THE PRODUCTION OF POLYSOMATOUS 
MONSTERS. 
The various theories regarding teratogenesis which had 
troubled mankind for so many centuries were finally exploded 
by naturalists, whose speculations gradually led them to ex- 
periment upon this subject. Anatomists and zoologists had 
deduced that the primary change lay in the egg about the 
time of fertilization, and we read in J. Miller,t Valentin? and 
Leuckart® that a double monster is due to division of the 
embryo-forming substance in the earliest stage of develop- 
ment. The experiments subsequently made by Gerlach,* 
Panum® and Dareste® upon chicks were negative in this 
respect, but the tradition has come down to us that polyso- 
matous monsters are produced by a process of splitting of the 
primitive streak. The more recent anatomists—Fol, Rauber, 
*Miiller, Meckel’s Archiv, 1828. 
*Valentin, Handworterbuch d. Physiologie, I, 1842. 
*Leuckart, De Monstris, Gottingen, 1845. 
“Gerlach, Doppelmissbildungen, 1882. 
*Panum, Entstehung der Missbildungen, 1860. 
°Dareste, Recherches sur la production de monstrosités, Paris, 1891. 
