18 MALL. [VoL. XIX. 
Born and O. Hertwig—who observed the developing egg and 
experimented upon it, were at first inclined to the theory that 
the first cause in the production of monsters is due to poly- 
spermy, but this has not been substantiated. 
The first reliable and valuable observations upon the pro- 
duction of double monsters were made by Vejdovsky,’ who 
noticed that the eggs of Lumbricus produce more monsters in 
warm than in cool weather, and he expressed the suspicion 
that they were produced by the change in temperature. 
Driesch® seized upon this idea, experimented upon sea-urchins’ 
eggs, and found by subjecting them to high temperatures that 
the cells, in the two-cell stage, separated, each growing into 
an individual, but, however, remaining connected with each 
other. Driesch had already shown that when the blastomeres 
of these eggs are fully separated by shaking, each grows into 
a whole embryo, and it was now clear to him that double 
monsters are produced by separating the blastomeres slightly, 
but still keeping them close enough together so that the inde- 
pendent embryos grow into each other’s bodies to form a 
double individual. 
By a very different method double monsters were also pro- 
duced by Loeb.® He subjected sea-urchin eggs to an equal 
mixture of sea-water and distilled water shortly after they 
had been fertilized. The rapid absorption of water caused 
many of the cell membranes to burst, and part of their proto- 
plasm escaped, which, however, remained connected with that 
inside of the membrane. All this took place before the nucleus 
had divided. Upon returning the eggs to normal sea-water 
cleavage began, and one of the first two nuclei wandered into 
the extruded protoplasm. Each nucleus with its protoplasm 
then became an embryo, and in case the embryo within the egg 
was not separated from the extra-ovate embryo by its active 
movements in the blastula and gastrula stage a double monster 
*Vejdovsky, Entwicklsg. Untersuchungen, Prag, 1890. 
*Driesch, Zeit. f. wiss Zool., LV, 1892. 
"Loeb, Biological Lectures at Woods Holl, 1893; Pfliiger’s Archiv, 
LV, 1894; Roux’s Archiv, I, 1895; and Studies in General Physiology, 
Chapter X, Chicago, 1905. 
