ACCELERATION OF RATE OF CELL DIVISION. 349 



in number, and in the complexity of the relationships which they 

 assume to each other. Later the intracellular processes of special- 

 ization assume a larger importance. The rapidity of organization, 

 therefore, is directly dependent upon the rate of division of the 

 constituent cells of the embryo. 



Differentiation, of course, is not synchronous with the division 

 of any particular cells. In fact, growth, and the processes which 

 accompany it, alternate in time with the successive mitoses. 

 Laughlin obtained evidence of this fact in his investigations on 

 the duration of the mitoses in the onion root tips. He pointed out 

 that " in a growing tissue, so far as the individual cell is concerned, 

 there is a definite alternation between permanent increase in bulk 

 and mitosis. Indeed, if bulk increase is largely anabolic and cell 

 division catabolic, as is most probably the case, then opposing 

 activities can not synchronize in the same cell each as a dominant 

 factor of activity. But synchronization of the same activities 

 among many neighboring cells is a different matter. This exists 

 and its degree determines the character of the pulsation observed 

 in rate of growth in actively growing tissues." From the stand- 

 point of a careful analysis, the processes which we usually speak 

 of as growth are to be distinguished among themselves ; mitosis, 

 increase in bulk, and differentiation are all involved. Mitosis and 

 increases in bulk can not usually take place in one cell at the same 

 time. Their independence in this relation, however, does not mean 

 that the three processes are not dependent on each other, for, of 

 course, they must maintain their proper balance or the organism 

 can not undergo further development. 



That differential rates of division in the various organs and 

 tissues of an embryo exist can easily be shown. A casual inspec- 

 tion of sections of any embryo will show that not all regions 

 exhibit cells in division with equal frequency. The mitotic ratio 

 (by which is meant the number of cells in some stage of mitosis 

 that can be counted in one hundred cells of the type in question) 

 varies within rather wide limits. From some investigations which 

 have been going on in this laboratory it appears that the ratio for 

 early chick embryos in their entirety varies from a fraction of 

 i per cent, to about 5 per cent. In any embryo many division fig- 

 ures may be found in some sections, while in others almost none 



