6 2 BIOLOGICAL LECTURES. 



egg is able to form a Pluteus provided that its mass is more 

 than one-eighth the mass of the whole egg. Now the question 

 arises, is development a function only of the mass or is it also 

 a function of the orientation of the protoplasm in the egg ? I 

 have shown in a former paper that the adoption of the theory 

 of Sachs leads us to the assumption that there exist in the 

 ovum chemically different substances which are not equally 

 distributed throughout the same. If this were true it might 

 make a difference which part of the protoplasm came under 

 observation in these experiments. But my experiments show 

 that in regard to the possibility of development every part 

 of the protoplasm of the sea-urchin's egg appears to be- 

 have as if it were isotropic. This is shown by the following 

 facts : If we rupture an egg the protoplasm flows out from the 

 place where the membrane is torn and the protoplasm that 

 escapes forms an independent embryo, provided the separation 

 is complete. It can further be shown that the place where the 

 membrane bursts bears no relation to the future embryo, or at 

 least to the first plane of cleavage. If we put normal eggs 

 that have just gone into the two-cell stage into diluted sea- 

 water we find that the cleavage plane may have any position in 

 regard to the place where the membrane bursts, as is evident 

 in Figs. 4-7. These figures are drawn by the camera from 

 eggs that were caused to burst after they had reached the two- 

 cell stage. As the extra-ovate develops in all cases in which it 

 is sufficiently large, we must conclude that the protoplasm of 

 the ovum may be considered an isotropic mass, as far as the 

 possibility of its development is concerned. 



8. The same is true of the nucleus. In my lecture of last 

 year I showed that the extra-ovate gets as a rule only a small 

 part, say one-fourth of the nucleus, but that this is sufficient to 

 enable it to develop into a normal embryo. Driesch has 

 shown, moreover, by experiments in which the eggs developed 

 under pressure that the nuclei may be considered isotropic as 

 regards their distribution in the cleavage cells. When he 

 brought eggs under one-sided pressure the distribution of the 

 nuclear material took place in a way that was different from 

 what happened under normal conditions. Nevertheless normal 



