The Development of the Frog 9 



contact with surrounding objects, and also to 

 hasten development by storing up heat, this 

 latter power being due to the fact that it per- 

 mits the heat of the sun to pass inward more 

 rapidly than it permits the reflected heat to 

 pass out again, with the result that the mass 

 of spawn, in the sunlight, is warmer than 

 the surrounding water. The jelly may also 

 protect the eggs from being eaten by other 

 animals. 



The egg itself consists of a black and a white 

 pole of approximately equal sizes, the dark 

 color of the black hemisphere being due to 

 the presence of a superficial layer of pigment 

 in that region. The origin of this pigment is 

 not clearly understood, but its purpose seems 

 to be that of absorbing the heat of the sun, 

 and in furtherance of this object the pigmented 

 half of the egg is of less specific gravity than 

 the other half, with the result that the dark 

 pole is always the upper one and the one in 

 which the segmentation takes place the more 

 rapidly. This automatic orientation is made 

 possible by the fact that the egg shrinks away 

 from its vitelline membrane, between which 

 and itself is secreted a small quantity of fluid 

 in which the egg may easily rotate. 



