^6 BIOLOGICAL LECTURES. 



paper Whitman has shown that from the standpoint of the 

 morphologist it would be equally erroneous to consider the cell 

 as the ultimate unit of living matter. 



2. In the experiments of Nussbaum, Gruber, and those who 

 followed them, only the qualitative side of our question has 

 been touched, in so far as they all tried to prove that neither 

 the nucleus without protoplasm nor the protoplasm without 

 nucleus can show phenomena of growth and regeneration. 



But there is a quantitative side of the problem, that is to 

 determine the limits of divisibility of living matter and the 

 order of magnitude of the smallest particle that can show all 

 the phenomena of life. Is such a particle of the magnitude of 

 a giant molecule of proteid substance, or of a micella, or a 

 combination of several micellae, or does it approach the magni- 

 tude of a cell } If we undertake to get an answer to this ques- 

 tion from the egg, we have to determine what is the size of 

 the smallest portion of an ^^^ which, if isolated, is able to 

 undergo normal development. I think we may take it for 

 granted that phenomena of development include all other func- 

 tions of living matter. Two methods by which the answers to 

 these questions might be obtained presented themselves. The 

 first method depends upon the fact that the ovum is divided 

 by segmentation into a continually increasing number of cells 

 which decrease correspondingly in size. We might isolate a 

 cell in different stages of the segmentation and see what is 

 the last stage from which a single cell can develop into a nor- 

 mal embryo. Such experiments have been made for another 

 purpose by Roux, Chabry, Driesch, Wilson, Hertwig, and others. 

 Driesch found that a single cell from the four-cell stage of a 

 sea-urchin's egg could still develop into a full embryo, but that 

 with one from the eight-cell stage development was apparently 

 no longer possible. But this method is not suited to give us 

 a reliable answer to our question, as we do not yet know 

 whether an isolated cell in the eight-cell stage of an embryo 

 is identical with the eighth part of an ovum before segmenta- 

 tion. It is at least possible that the ovum during segmentation 

 is divided up into cells or regions of chemically diverse ma- 

 terials. It is further possible that the metabolic processes 



