80 PROTOPLASM 



the organism. The influence of these last environmental factors 

 is illustrated by the classical experiment which involves the 

 growth of a single cell, or hlastomere, separated from a young frog 

 embryo. It has been found that if the cells of a 2-celled frog 

 embryo are separated from each other, each will develop into a 

 normal frog. This is also true even of the 4-, the 8-, and the 

 16-celled stage; that is to say, each of the 2, 4, 8, or 16 cells of an 

 embryo is potentially capable of producing a whole frog. Each 

 of these cells, before separation from the others, was destined to 

 become a part of a frog, but now, after separation, their destiny 

 is changed, and each cell gives rise to all the tissues of an entire 

 frog. (In actual experiments, not all cells grow when separated; 

 it is necessary that only one of the embryonic group should grow 

 to prove our point.) 



The problem of the destiny of a cell arises wherever many 

 types of tissues have a common origin, which is rather generally 

 true throughout nature. A single initial cell in the growing 

 point of certain roots and stems (Fig. 20) gives rise to all the 

 diverse tissues of the mature root or stem. It may be said that 

 cell type is determined by function, but how does the cell when 

 young "know" what its function is to be, so that it may develop 

 into the kind of cell which it must become in order to fulfill 

 that function? The most satisfactory solution of the problem 

 is that which places the responsibility for the function and 

 destiny of the cell upon its relative position in the organism. Any 

 other cell would, if taken young, carry on the same function 

 if it found itself in the same position. In tissue cultures, relative 

 position has no meaning because there is little differentiation, and 

 that little is determined by origin, nutrition, and time. 



Another peculiarity of tissue-culture cells is their occasional 

 multinuclear condition. The rule among cells is one nucleus 

 apiece (exceptions have been cited). In culture, it is not uncom- 

 mon to find cells which have six or even a dozen nuclei. H. D. 

 Fell, W. H. Lewis, and others have described how this multi- 

 nuclear condition may arise. Sketches by Fell and Andrews 

 (Fig. 64) show how a binucleate cell results from incomplete 

 division. At 3 :07 o'clock, the chromosomes are on the equatorial 

 plate; i.e., division is half complete; at 3:10, the chromosomes 

 have reached their respective poles; at 3:20, nuclear division is 

 complete, and cell division well underway; at 4:07, the two 



