330 CHARLES PACKARD 



Kite the protoplasm has increased its water holding power so 

 that the whole egg becomes soft and fluid. A further evidence 

 of the softened character of the protoplasm is shown in figure 2, 

 The peripheral layer is pulled in abnormally under the influence 

 of the aster. 



Cleavage occurs in a fair proportion of these eggs, but it is 

 abnormal. After several divisions the embryo resembles a 

 heap of shrunken cells. Occasionally the cells put out cilia. 



Eggs which are treated with both slow and rapid beta rays for 

 varying periods before insemination do not show so marked an 

 increase in the width of the perivitelline space. But evidently 

 they have been greatly injured since in most cases division is 

 entirely inhibited. Those eggs which divide at all do so in a 

 very abnormal manner. 



These facts indicate that the rapid beta rays acting at a dis- 

 tance of 50 mm. are not strong enough to produce any marked 

 abnormality in development, but cause a pronounced retar- 

 dation in the rate of development. Acting at 4 mm. distance 

 they affect chiefly the peripheral layers of protoplasm. When 

 both slow and rapid rays are used cell division is usually in- 

 hibited altogether. 



2. Experiments on Arbacia. In the experiments on the sea 

 urchin I used both fertihzed and unfertihzed eggs, observing the 

 proper precautions to have all the conditions for growth as 

 favorable as possible. The general effect of the rapid beta 

 rays, acting at a distance of 50 mm. is a retardation, the slowing 

 down commencing almost at once in the case of the fertilized 

 eggs. In text figure B the amount of retardation is shown. The 

 curves were made from an average of many observations made 

 on different lots of eggs. In each case the eggs were exposed 

 for 40 minutes immediately after insemination. It will be seen 

 that the control eggs in these experiments begin to divide about 

 75 minutes after insemination, and that all are in the two cell 

 stage about 10 minutes after they begin to divide. The radiated 

 eggs are much retarded both in beginning to divide and in at- 

 taining the complete two cell stage. At the latter time the 

 controls have begun to divide for the second time. The pause 



