EARLY DEVELOPMENT IX THE FROG. 



3 2 5 



above the middle of the grey crescent and spreads equatorially 

 and apically to join the area where dissolution first began. At 

 this time the disintegrated area forms, roughly, a triangular 

 figure whose apex is the point where disintegration first began, 

 and whose base is the equatorial region immediately above the 

 gray crescent. Disintegration then spreads more or less sym- 

 metrically from the apex and sides of the triangle over the rest 

 of the animal pole, and from the base to involve the grey crescent 



fl 



C 



D 



FIG. 4. L T nsegmented egg seen from above (apical pole) showing different stages 

 in disintegration after twelve days' exposure to a temperature of o C. Dis- 

 integrated areas are represented by stippling. The dotted circle represents the 

 boundary between pigmented and unpigmented hemispheres. The dotted crescent 

 outlines the gray crescent. The cross marks the animal pole. 



region. Fig. 6 illustrates the process just described. The history 

 and treatment of the eggs is given in the legends to the figures. 

 (c) Gastrula Stages. Eggs in an early gastrula stage, when 

 placed in lethal concentrations of a toxic agent, always begin 

 to disintegrate first in the dorsal lip region, and shortly afterward 

 in the same meridian about 120 to 130 above the blastophore. 

 From this upper point dissolution of the surface cells proceeds 

 down the meridian and meets the disintegrated area of the dorsal 



