20 DEVELOPMENT OF THE FROG'S EGG [Cn. II 



and protected from injury from without. The slime protects 

 them from water-snails that will eat the eggs if they are shelled 

 out from the jelly. The jelly may also protect them against 

 water-birds. The eggs and young tadpoles seem, however, in 

 themselves to be distasteful to certain Crustacea (Bernard and 

 Bratuschek, '91). 



This jelly has the physical peculiarity of allowing the sun's 

 rays to pass through, but hinders reflection of the rays from 

 the interior to the outside. The result is that in the sunlight 

 the mass of eggs is at a higher temperature than the surround- 

 ing water, and as the eggs of many frogs are laid in the early 

 spring, when the water is quite cool, this property of the jelly 

 helps to hasten their development. 



Hertwig ('77) thought that a change takes place in the inte- 

 rior of the egg after fertilization, so that a difference in the 

 specific gravity of different parts of the egg is brought about. 

 Schultze ('87), however, pointed out that at this period the 

 egg contracts slightly from its vitelline membrane, and between 

 the egg and its membrane a fluid collects, that is probably 

 squeezed out of the egg itself. The egg, freed from its inner- 

 most coat which held it in place, then rapidly orients itself with 

 respect to gravity. Unfertilized eggs will also, after a time, 

 slowly rotate, and in these it can be seen that the separation 

 of the egg from its membrane is less perfect than in fertilized 

 eggs. "At the moment when the ovum is expelled from the 

 body, the envelope is merely a thin gelatinous layer, its entire 

 diameter being equal only to about one-sixth of the diameter 

 of the yelk. After it has been one minute in water, and begun 

 to imbibe and expand, it is then equal to about one-fourth of 

 the diameter of the yelk. At the end of tivo minutes it is en- 

 larged to one-third, and in three minutes, to one-half the diame- 

 ter of this body. In four minutes, it exceeds three-fifths, and 

 in six minutes, two-thirds, and it continues to imbibe fluid and 

 expand at the same rate, until, at from ten to fifteen minutes, 

 it very nearly equals in thickness the whole diameter of the 

 yelk ; and at half an hour it is one-fourth greater than this. 

 At the end of three hours the membranes have acquired nearly 

 their full size." 



"The expansion of the envelope is greatly retarded at the 



