PRODUCING ACCRETE EMBRYOS FROM ONE EGG 307 



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 



