DEVELOPMENT 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 



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. Sp. N., Sperm nuclei. 



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.) 



