LIMITS OF DIVISIBILITY OF LIVING MATTER. 6 1 



Pluteus. It is evident, taking into account the difference in 

 the method of experimentation, that the limit of divisibility 

 determined by Driesch coincides as nearly as could be expected 

 with the limit that we have found. Hence Driesch's and 

 Wilson's experiments do not force us to assume that during 

 the early stages of development qualitative changes take place 

 which prevent a single cell of the eight-cell stage from de- 

 veloping into a complete embryo. But the same fact can be 

 proved in another way. If we let an Qgg develop normally and 

 bring it into the diluted sea-water as soon as it reaches the 

 eight, sixteen, or thirty-two cell stage, the membrane bursts and 

 part of the contents flow out, just as happens in the unseg- 

 mented ovum, only with the difference that the extra-ovate 

 consists of a greater number of cells. In this case also, as in 

 the unsegmented ovum, the development depends upon the 

 quantity of material. Fragments that are larger than one- 

 eighth of the whole ovum may develop into Plutei ; smaller 

 fragments will only reach the blastula or gastrula stage. If 

 the early segmentation produced not only an increase in the 

 number of cells but also a definite qualitative differentiation, 

 we should expect that from a small isolated group of cells — 

 say three — from the thirty-two-cell stage there would result an 

 irregular mass of tissue which later on might be transformed 

 by regeneration into a normal embryo. But the embryo is not 

 produced in this way. Such a mass of cells develops directly 

 into a normal blastula and either remains in this stage or is 

 transformed into a gastrula. (This agrees with the results of 

 similar experiments of Driesch.) There is another method of 

 determining whether or not the embryo undergoes differentia- 

 tion during the early stages of segmentation. If it does, the 

 differentiation must be accompanied by chemical changes. 

 But if chemical changes took place the physiological reactions 

 would change too. I made experiments on fish embryos and 

 found that in the first stages of segmentation such changes do 

 not take place. I made such experiments on sea-urchins also 

 and with the same result. 



7. It is clear from the preceding that when it consists partly 

 of nuclear material, a piece of protoplasm from a sea-urchin 



