150 Organisms from Eggs 



infusorian is needed to maintain its life. The question 

 then arose: How small a fraction of the original cell 

 is required to permit the full maintenance of life ? The 

 writer tried to decide this question in the egg of the 

 sea urchin. He had found a simple method by which 

 the eggs of the sea urchin (Arbacia) can easily be divided 

 into smaller fragments immediately after fertilization. 

 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 equal parts of distilled water, the egg takes up water, 

 swells, and causes the membrane to burst. Part of the 

 protoplasm then flows out, in one egg more, in another 

 less. If these eggs are afterward brought back into 

 normal sea water those fragments which contain a 

 nucleus begin to divide and develop. 1 It was found 

 that the degree of development which such a fragment 

 reaches is a function of its mass; the smaller the piece, 

 the sooner as a rule its development ceases. The small- 

 est 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 t a great 

 discrepancy in the calculation of the mass. Driesch's 



1 Loeb, J., Arch. d. f. ges. PhysioL, 1893, lv., 525. 



