54 CHARLES PACKARD. 



centrosome being surrounded by a well-marked aster. Finally 

 the sperm grows into a vesicular form resembling the egg vesicle 

 with which it soon comes in contact. Actual mingling of the 

 chromatin elements does not occur until the metaphase of the 

 first cleavage. Up till that time the chromosomes from each 

 nucleus can easily be distinguished. Before the chromosomes 

 have condensed into their characteristic shape there can be seen 

 in the cleavage nucleus a number of chromatin nucleoli, probably 

 derived from the chromosomes. During cleavage these lag be- 

 hind, staying in the cleavage plane. The achromatic stuctures 

 are very prominent, the asters remaining even after the com- 

 pletion of cleavage. 



OBSERVATIONS ON THE LIVING EGGS. 



To test the effect which gradually increasing amounts of radia- 

 tion will produce in the rate of cleavage and in the production of 

 abnormal cleavages it is necessary to employ throughout any one 

 experiment eggs taken from a single female, for it was found that 

 eggs from different specimens varied in the rate of cleavage and 

 in other ways. In each experiment a sufficient quantity of eggs 

 was exposed, and from this number a few were taken off at 10- 

 minute intervals and fertilized in finger bowls. The proportion 

 of cleaving eggs, compared w r ith the controls, and the number of 

 abnormal cleavages were recorded. This experiment was re- 

 peated on different days, and with eggs derived from different 

 females. 



It was found that brief radiations do not affect the rate at 

 which the polar bodies are extruded. Indeed, prolonged radi- 

 ations have little effect, for only in a few instances could any 

 unusual condition be found. A cytological study showed that as 

 long a treatment as 80 minutes causes very few abnormal polar 

 divisions. Cleavage is much retarded, the most pronounced 

 retard being seen in eggs radiated for 30 to 40 minutes previous 

 to insemination. Longer exposures result in a more normal rate. 

 This unexpected result is due to the fact that eggs which have 

 been radiated for a long time are almost all polyspermic and such 

 eggs always divide rapidly, sometimes ahead of the controls, and 

 always into three or more parts. 



