ACCELERATION OF RATE OF CELL DIVISION. 357 



been started is hastened to completion, and in almost every case 

 that state has been reached by the time the egg can be taken from 

 under the tube and examined under the microscope." During the 

 depression phase following exposure of this egg a second stimu- 

 lation may be brought about by reexposure, but the extent of the 

 acceleration is less and the second depression follows more rapidly. 



A later experiment of slightly different character, but bearing, 

 I think, upon this general problem, may be noted here. The 

 writer exposed pepsin and diastase to x-rays and found that a 

 short radiation accelerated the activity of both these preparations 

 of enzymes, that a longer exposure inhibited the activity, and that 

 between these two strengths there lies a point at which radiation 

 is non-effective. Exactly similar results were obtained by the 

 writer and Miss Woodward upon exposing the cell extractive of 

 Echinoderm eggs, " fertilizing to x-radiation. The parallel be- 

 tween the behavior of these enzymes and extractives and that of 

 mitotic processes under the influence of radiation is probably not 

 without its significance. 



Certain substances derived from internal glands are thought to 

 favor growth, chief among them being thyroid constituents. It 

 might be presumed from this effect that these substances would 

 also stimulate cell division. Nowikoff, Shumway, and also Bud- 

 dington and Harvey have investigated the effect of thyroid upon 

 ciliates and have arrived at essentially the same conclusion. 

 Shumway added a small amount of thyroid emulsion to the hay 

 infusion in which Paramcecium- aurelia were growing and a sharply 

 marked increase in the division rate resulted. He reports as fol- 

 lows : 'Thyroid substance fed to Paraincvciitin aurelia or cau- 

 datuni, either as an emulsion of raw thyroids or as a suspension 

 of the commercial powder, produces a constant and significant in- 

 crease of 65 per cent, in the rate of division over that observed in 

 the common laboratory hay-medium infusion. The thyroid is the 

 only one of the internally secreting glands that produces this effect. 

 Boiling the thyroid produced no change in the reaction. lodo- 

 thyrin and iodine fail to produce the thyroid effect. Parauuvcia 

 after prolonged thyroid treatment revert to the normal division 

 rate when returned to the control medium. The life-history 

 curves of the thyroid-treated lines show the same depression 



