Propvucine ACCRETE EMBRYOS FROM ONE Eaea@ 807 
fully. Herbst has apparently observed the same thing in 
eggs which he raised in salt solutions which had a qualita- 
tively abnormal constitution. The experiments of Driesch, 
who found that when the separate cleavage cells are isolated 
in the four-cell stage, each of the cleavage cells is still able 
to develop into a completely normal embryo, can also be 
explained in this way. When the mass of the protoplasmic 
sphere becomes too small, the formation of a blastula, of 
course, becomes impossible on geometrical grounds alone, 
for the size of the cells probably does not fall below a certain 
minimal volume during cleavage, 
Even though these experiments can leave no room for 
doubt that an embryo may arise equally well from every part 
of the protoplasm and from every part of the nucleus, as soon 
as these parts are isolated to a certain degree, and can 
assume a spherical or ellipsoid form, the fact nevertheless 
remains that in many eggs conditions are apparently present 
which determine the position of the median plane of the 
embryo and the further orientation of the various parts of 
the egg. These circumstances may, however, be purely sec- 
ondary in character, and may depend upon the mass and 
distribution of the nutritive yolk, the position of the micro- 
pyle, and similar secondary conditions. On the other hand, 
my experiments do not seem to agree with the assumption 
that each part of an egg can give rise only to a certain part 
of the embryo. 
5. We must now raise the question whether the forma- 
tion of twins or double embryos in mammals can come about 
in a way similar to that described in these experiments. 
Driesch isolated individual cleavage spheres by rupturing 
the membrane through shaking. It is probably impossible 
that an egg can be ruptured in this way in the Fallopian 
tubes of a mammal. On the other hand, it may be possible 
that the scheme of my experiments corresponds to natural 
é 
Digitized by Microsoft® 
