8o PROBLEMS OF FERTILIZATION 



in the periblast which forms extra-embryonic tissue. 

 It may be, however, that this conclusion is due to con- 

 fusion with other nuclei of the periblast derived from 

 the segmentation nucleus. 



The peripheral migration of the supernumerary 

 nuclei furnishes an interesting problem. Miss Blount 

 has suggested in her study of these nuclei in the pigeon 

 that the phenomenon can be understood on the simple 

 assumption that migration of the sperm nuclei is al- 

 ways into unfertilized protoplasm, i.e., such as has not 

 yet been modified by the sperm nuclei; the central 

 protoplasm surrounding the egg nucleus is thus modi- 

 fied by the first sperm nucleus that moves into it, but 

 the peripheral protoplasm is still virgin soil. 



There would thus seem to be no advantage connected 

 with physiological polyspermy; at the most it is harm- 

 less, and merely represents a condition in which the 

 final determination of the successful spermatozoon is 

 completed within the egg. Such eggs have in some 

 way overcome the usually harmful effects of polyspermy. 

 Bonnevie (1907), however, is of the opinion that in 

 Bryozoa at least it is significant for the maintenance 

 of the organism; she suggests that the supernumerary 

 spermatozoa furnish extra-nuclear chromatin of physio- 

 logical importance. There is, however, no adequate 

 foundation for such a view at present. 



2. Pathological polyspermy. — Eggs normally mono- 

 spermic may be entered by more than one spermatozoon 

 if they are allowed to become stale before insemination; 

 the same result may be attained by exposing them to the 

 action of various injurious substances, such as chloroform, 

 chloral hydrate, cocaine, nicotine, strychnine, quinine. 



