3O DYNAMICS OF LIVING MATTER 



tion into nucleus and protoplasm is definitely and permanently lost, 

 is unable to accomplish its functions.* 



The observations of Nussbaum and those who repeated his experi- 

 ments showed that although two different structures are required, not 

 the whole mass of an Infusorian is needed to maintain its life. I tried 

 to solve the question as to how small a fraction of the original cell must 

 be preserved in order to maintain life in the sense of the definition 

 given at the beginning of these lectures. Will the smallest possible 

 element be of the order of an aggregate of a few molecules, or will it 

 be of the order of a small fraction of the original mass ? I tried to decide 

 this question in the egg of the sea urchin, immediately after its fertiliza- 

 tion. The egg divides and reaches successive larval stages, first a bias- 

 tula, then a gastrula, and finally a pluteus stage. Without especial 

 efforts the eggs cannot be raised beyond this stage in the laboratory. 

 I had found a simple method by which the unsegmented eggs of the 

 sea urchin (Arbacia) can easily be divided into smaller fragments. 

 When the egg is brought from five to ten minutes after fertilization 

 (long before the first segmentation occurs) into sea water which has 

 been diluted by the addition of an equal part of distilled water, the 

 egg takes up water and the membrane bursts. Part of the protoplasm 

 then flows out, in one egg more, in another less. If these eggs are after- 

 ward brought back into normal sea water those fragments which con- 

 tain a nucleus begin to divide and develop. f In this case the degree 

 of development such a fragment reaches, is clearly a function of its 

 mass; the smaller the piece, the sooner on the whole its development 

 ceases. The smallest fragment which is capable of reaching the pluteus 

 stage possesses the mass of about one eighth of the whole egg. Boveri 

 has since stated that it was about one twenty-seventh of the whole 

 mass. Inasmuch as only the linear dimensions are directly measurable, 

 a slight difference in measurement will cause a great discrepancy in 

 the calculation of the mass. 



These results are in harmony with experiments made by Driesch J 

 for a different purpose. Driesch isolated the first blastomeres of the 

 segmented egg of a sea urchin by shaking the egg and thus bursting 

 its membrane. He found that an isolated cell of the two- or four-cell 

 stage of the egg of a sea urchin is still capable of developing into a 

 normal pluteus, but that an isolated blastomere of the eight- or sixteen- 

 cell stage no longer possesses this power. This experiment, however, 



* It must not be overlooked that in bacteria and the blue algae no distinct differentia- 

 tion into nucleus and protoplasm can be shown. To these organisms, therefore, the experi- 

 ments of Nussbaum cannot be applied. 



t Loeb, P/l'tiger's Archiv, Vol. 55, p. 525, 1893. 



J Driesch, Zeitsch. f'iir wisse nschaftli sche Zoologie,\o\. 53, 1891. 



