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 
Fic. 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 dise. x 2000. 10.40 p.m. (After 
Harper.) 
