DEVELOPMEXT PRIOR TO LAYING 



37 



The entrance of several spermatozoa appears to be character- 

 istic of vertebrates with large ova; thus for instance, it has been 

 described in selachii, some amphibia, reptiles, and birds. Such 

 a condition is known as polyspermy; it is normal in the forms 

 mentioned, but occurs only under abnormal conditions in the 













iev'.'.V: ■■■;•- ■; 





a^': 





mm: 



fi:/::v 















'-iii 'Sp.N: 



:■•.•:) 





■^SA:-^;;; 





"^^•-^^^'-■y^'^^^'■t^-•-"^^' '■^•^•f^i •"•—'■"''' '^'''^^^ 



mm 





'.-'•'•J- 



Fig. 14. — Horizontal section of the germinal disc of a pig- 

 eon's ovum immediately after ovulation, x 125. (After 

 Harper.) 



N., Nucleus, probably first maturation spindle, p. r., 

 Polar ring. 8p. N., Sperm nuclei. 



K" 





\^ 



f^ 



1,5" '^.V^-^ 



-J 



Fig. 15. — Vertical section of the pigeon's egg showing germ nuclei 

 (pronuclei) in the center of the disc, x 2000. 10.40 p.m. (After 

 Harper.) 



