EFFECT OF RADIUM ON DEVELOPMENT OF CH/ETOPTERUS. 65 



spheres are pushed away from the advancing sperm so that the 

 latter lies in an area of clear protoplasm. In respect to the sperm, 

 therefore, the egg protoplasm reacts normally. Only the egg 

 nucleus fails to migrate. As the duration of exposure increases 

 the nucleus moves a shorter and shorter distance. In other 

 words, the greater the injury to the chromatin, the smaller is the 

 distance through which it moves. This is shown in Fig. 1 1. The 

 egg nucleus under the polar bodies has formed into chromosomes 

 and chromatin masses, of which the most abnormal in appearance 

 is still in its original position while the rest have moved down to a 

 position normally occupied by the egg nucleus at this stage. This 

 condition suggests that the chromatin is concerned in its own 

 movement. 



This idea is supported by the behavior of the radiated chro- 

 matin shown in Figs. 8 and 9. In Fig. 8 the radiated chromatin 

 in the form of chromosomes is left behind during the division of 

 the sperm nucleus. One chromosome is normal in appearance 

 but the spindle fiber attachment is abnormal; instead of being 

 median it is terminal. In this instance there is an ineffectual 

 attempt at movement. In Fig. 9 no spindle fibers are found 

 attached to the irregular masses of egg chromatin, and the masses 

 do not move with the sperm chromosomes. 



According to Chambers ('17) the spindle fibers are lines of 

 stress or of protoplasmic flow ; they are not formed structures. As 

 such they are probably produced as a result of the interaction of 

 the chromosomes on the egg protoplasm. When the chromatin 

 is rendered abnormal by the radiations it fails to stimulate their 

 production and therefore does not move with the sperm chromo- 

 somes to which spindle fibers are attached. If the fibers develop 

 under the influence of the chromosomes, there is no need to 

 assume the presence of contractile fibers in the protoplasm which 

 serve to pull the chromosomes toward the poles. They are not 

 bodies which are passively pulled about in the cell, but are ac- 

 tively concerned in their own movement. 



SUMMARY. 



i. Radiation of the unfertilized eggs of Chcetopterus results in 

 injurious effects which do not become manifest until after the 



