THEORIES OF DEVELOPMENT 25 I 



insufficiency of the cell theory, and think that cell boundaries play 

 no important part in the development, but that the embryo develops 

 as a whole. This has seemed to me to be the more probable view in 

 the light of certain results of experimental embryology. Driesch, in 

 later papers, has also opposed Hertwig's idea, and Wilson in his book 

 on The Cell has also, to a certain extent, adopted this point of 

 view. The formation of a typical larva in the sea-urchin and in am- 

 phioxus out of one-half or one-fourth the whole number of cells 

 demonstrates, I think, the insufficiency of the cell-unit hypothesis. 

 The discovery of continuous protoplasmic connections between neigh- 

 boring cells, and the formation of new protoplasmic connections 

 between all regions, as found by Mrs. G. F. Andrews, 1 gives us a 

 basis of fact on which to rest the hypothesis of the embryo being a 

 whole structure. This view meets with no great difficulty on the 

 grounds that the nuclei are distinct centres of metabolic activity, for 

 we know at present so little of what sort of action takes place between 

 the nucleus and the protoplasm that we cannot rest our argument on 

 any demonstrable relation. 



The discovery that pieces below a certain minimum size are inca- 

 pable of producing a whole organism is of capital importance. It 

 has been pointed out that pieces of the egg of the sea-urchin less than 

 one-sixteenth of the whole do not produce even the gastrula stage. 

 In amphioxus the one-eighth blastomere seems to be near the lower 

 limit of development. It has also been found that there is a lower 

 limit for pieces of adult organisms below which they do not regen- 

 erate. This has been shown for hydra, tubularia, planarians, and 

 stentor, and is probably true for all forms. This result is especially 

 interesting in those cases in which the parts contain all the elements 

 necessary to produce a new organism, and come from parts of the body 

 that are totipotent in these respects. It seems certain that the 

 lack of power of development in these cases is due entirely to the 

 smallness of the piece. We can express the idea in another way by 

 stating that a certain volume is necessary in order that a piece may 

 produce the typical organization. This conclusion is important as 

 showing that the organization is something enormously large as com- 

 pared with the size of the chemical or physical molecules, and even 

 of the crystal molecule. The size of a piece that is at the lower 

 limit of organization is also very much larger than the smallest cells 

 of which the embryo is made up, and this relation is a point in favor 

 of the view that the organization is not simply the resultant of the 

 interaction of the cells, but is something much larger than these cells ; 

 and we may even go further, I think, and add that it dominates the 

 cells rather than is controlled by them. 



11897. 



