No. 2.] THE EGG OF BUFO LENTIGINOSUS. 325 



little finer than the other. The two fibers are united by a thin 

 transparent membrane which narrows at the posterior end 

 where the two fibers run together. The longer fiber extends 

 some distance beyond the place of union and forms a pointed 

 termination of the tail. The delicate membrane between the 

 fibers may be easily destroyed by reagents, leaving the fibers 

 completely separated, so that, as stated by La Valette St. 

 George ('76), the spermatozoon appears to have two thread- 

 like tails which are in no way connected after they emerge 

 from the middle-piece. 



No portion of the tail colors intensely with either iron-haema- 

 toxylin or the combination stain, although with either stain the 

 fibers take a faint bluish tint, while the membrane remains 

 colorless. 



V. Fertilization of the Ovum. 



I. The Penetration of the Spermatozoon into the Egg and 

 the Formation of the Astrosphere. 



The pointed head of the spermatozoon so quickly bores its 

 way through the thick membrane surrounding the egg that fer- 

 tilization is usually effected within three or four minutes after 

 the Qgg is laid. The spermatozoon can penetrate at any point 

 of the upper hemisphere of the &gg, but I have not been able 

 to confirm the result of Fick, Jordan, and Roux that it can also 

 enter any portion of the lower hemisphere. 



Kupffer ('82) has observed the entrance of the spermatozoon 

 into the living ^gg of Bufo variabilis and describes it thus : 

 When the head of the spermatozoon touches the egg-mem- 

 brane, the protoplasm of the ^gg draws back slightly at the 

 point of contact but quickly returns again to its first position. 

 The period of penetration of the spermatozoon from the 

 moment of contact of the sperm-head until the spermatozoon 

 disappears into the Qgg lasts in some cases from one to one 

 and a half minutes, in other cases but three-fourths of a 

 minute. 



In preserved specimens of newly fertilized eggs of Bufo len- 

 tiginosus there is usually a slight depression in the surface of 



