66 CHARLES PACKARD. 



extrusion of the polar bodies, following insemination with normal 

 sperm. That two normal mitotic divisions should intervene after 

 radiation and only then that the injury to the chromosomes be- 

 comes apparent can be explained by the fact that the two longi- 

 tudinal divisions preparatory to polar-body formation have been 

 completed. Only when the chromosomes again "grow" in 

 preparation for a new division do abnormal characters manifest 

 themselves. 



2. If the radiation is brief (20 to 30 min.) the germ nudei fuse 

 normally but the radiated egg chromatin fails to form normal 

 chromosomes. No spindle fibers develop from these abnormal 

 chromatin masses, and they remain in the cleavage plane during 

 mitosis. If the radiation is longer (35 to 50 min.) the chromo- 

 somes, after the extrusion of the polar bodies, develop abnormally 

 and fail to fuse with the sperm nucleus. In both cases the sperm 

 nucleus divides and development is androgenetic. 



3. During cleavage the most injured chromatin does not move 

 and lacks spindle fibers. This suggests that the fibers are 

 normally developed under the influence of the chromosomes and 

 that the latter are concerned in their own movement. 



LITERATURE CITED. 

 Allyn, H. M. 



'12 The Initiation of Development in Chaetopterus. BIOL. BULL., Vol. 24. 

 Chambers, R. 



'17 Microdissection Studies, II. The cell aster. Journ. Exp. Zool., Vol. 23. 

 Conklin, E. G. 



'17 The effect of centrifugal force on the structure and development of the egg 



of Crej.idula. Journ. Exp. Zool., Vol. 22. 

 Herlant, M. 



'n Etude sur les bases cytologiques clu mecanisme de la parthenogenese 



experimentale chez les Amphibiens. Arch, de Biol., Tm. 28. 

 Hertwig, G. 



'n Radium bestrahlung unbefruchteter Froscheier. Arch. f. mikr. Anat., 



Bd. 77- 

 '12 Die Schicksal des mit Radiumbestrahlten- Spermachromatin im Seeigelei. 



Arch. f. mikr. Anat., Bd. 77. 

 '13 Parthenogenesis bei Wirbeltieren, hervorgerufene durch artfremden radium- 



bestrahlten Samen. Arch. f. mikr. Anat., Bd. 81. 

 Hertwig, G., and P. 



'13 Beeinflussung der mannlichen Keimzellen durch chemische Stoffe. Arch. 



f. Mikr. Anat., Bd. 83. 

 Hertwig, P. 



'13 Das Verhalten des mit radiumbestrahlten Spermachromatins im Froschei. 

 Arch. f. mikr. Anat., Bd. 81. 



