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 







m 





i'^1'--.'-. 



WM^S-y--]':-^^^^^^^^^^^ 





'^^■' 



'-.-.■.■■ 













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. 



V^ 



/■'itft AV^, 



W 





.--o 







"1 



.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.) 



