22 DEVELOPMENT OF THE FROG'S EGG [Cm. II 



the original egg-nucleus itself, which was squeezed out of the 

 egg at this time. Now, however, since we know the complete 

 history of the nucleus during this period, the suggestion of its 

 entire loss by the egg does not call for serious criticism. 



Entrance of Spermatozoon and Copulation of 

 Pronuclei 



The sperm of the male is poured out into the water, and 

 probably over the eggs themselves at the moment when they 

 are laid, and the spermatozoa begin at once to bore into the 

 jelly of the egg-mass (Fig. 10). 



Kupffer has described the entrance of the spermatozoon into 

 the eggs of Bufo variabilis. When the head of a spermatozoon 

 touches the egg-membrane, the protoplasm of the egg draws 

 back slightly at the point of contact, but quickly returns again 

 to its first position. The period of penetration of the sperma- 

 tozoon from the moment of contact of the sperm-head until the 

 spermatozoon disappears into the egg^ lasts in some cases from 

 one to one and a half minutes, in other cases only three-fourths 

 of a minute. Several spermatozoa were observed by Kupffer 

 to enter each egg. 



Other spermatozoa reach the egg-membrane, but do not seem 

 to be able to enter the egg. In the regions where these sper- 

 matozoa lie, the surface of the egg rises up in small protuber- 

 ances. This process occurs about fifteen minutes after the first 

 spermatozoa have entered, and lasts about one or two minutes, 

 after which the protuberances sink back into the egg. The 

 spermatozoa in the regions of the protuberances are left outside 

 the egg-membrane. This peculiar phenomenon is described by 

 Kupffer as a counter demonstration of the egg against those 

 spermatozoa that have not been able to enter. Eggs that have 

 been artificially fertilized show, when cut into sections, that one 

 hour after fertilization a dark pigmented streak is formed, 

 reaching from the pigmented coating of the egg into the yolk- 

 mass. The process takes place in the upper or dark hemi- 

 sphere, and regularly at one side of the centre of the dark field 

 near to the edge of the white border. The streak takes a 

 somewhat oblique course toward the centre of the egg. At 



