52 CHARLES PACKARD. 



polyspermy is rare, and the control eggs develop with perfect 

 regularity if kept under appropriate conditions. The number of 

 chromosomes is small, the haploid number being nine, and the 

 achromatic structures are unusually large. The most valuable 

 feature is the fact that the unfertilized eggs, when placed in sea 

 water, proceed in their development as far as the metaphase of 

 the first maturation division, at which time further development 

 ceases. Now it has been shown (Packard, '16) that chromatin, 

 when in its most condensed stage at the time of the metaphase, is 

 particularly susceptible to radium radiations. A short exposure 

 therefore is sufficient to injure the chromatin without harming 

 the protoplasm. This is an important point, for I have shown 

 (Packard, '15) that prolonged radiations injure the protoplasm 

 and bring about a very abnormal type of development quite 

 different from that which follows the injury of the chromatin 

 alone. A further advantage possessed by these eggs is their 

 extremely labile condition. As Miss Allyn ('12) showed, they 

 can be stimulated to develop parthenogenetically by a great 

 number of agents. I have found that prolonged exposures will 

 not prevent it from developing when fertilized by a normal sperm. 



The metaphase of the first maturation division in the egg of 

 Chcetopterus is reached about twenty minutes after the unferti- 

 lized eggs are put in sea water. In carrying out an experiment 

 I allowed the eggs to remain for this period in sea water and 

 then exposed them to the radium. A quantity of eggs is gathered 

 into a compact mass in the center of a watch glass containing a 

 measured quantity of water. The tube containing the radium 

 bromide, having the strength of 50 mg. of pure bromide, is now 

 held over the center of the mass of. eggs at a distance of ^e inch 

 above it. It is obvious that the eggs did not all receive exactly 

 equal amounts of radiation, but the difference in amount which 

 each received is very slight. 



The alpha rays are not able to penetrate the thin walls of the 

 glass tube in which the radium salt is held and are therefore not 

 responsible for any of the effects obtained. Probably some of the 

 slower beta rays are also cut off by the glass or by the sea water 

 over the eggs; but the great majority of beta rays and all of the 

 gamma rays are able to reach the eggs. 



