86 OUTLINES OF CHORDATE DEVELOPMENT 



side on which the gray crescent appears, i.e., on the side of the 

 animal hemisphere farthest from the fertilization point. Nor- 

 mally this arrangement is sufficiently marked before the rota- 

 tion of the egg is completed, so that when the egg comes to a 

 position of rest the animal pole is not turned exactly upward. 

 In most species of frogs the egg axis is inclined out of the 

 vertical about thirty degrees; and of course at the same time 

 the margin of the pigmented area is similarly tilted out of the 

 horizontal and the gray crescent lies on that side which is the 

 higher (Fig. 29) . In the egg at rest, therefore, we may describe 

 a definite plane which is vertical and includes both the gravi- 

 tational and the polar axes; from the mode of determination 

 of the position of the gravitational axis this plane also includes 

 the fertilization point and meridian, and the penetration path. 



While the egg is in this position the streaming and rearrange- 

 ment of its materials is completed, and since the specific gravity 

 of the different materials is concerned in. the rearrangement, 

 it takes place finally with reference to the direction of gravity 

 in the egg at rest. The final steps in the determination of 

 the structure of the unsegmented egg, therefore, take place 

 with reference to this gravitational plane, which thus becomes 

 the plane of bilateral symmetry of the egg structure. The 

 symmetry of the egg is expressed externally at this time only 

 by the gray crescent which is equally divided by the plane of 

 symmetry, but this is merely an indication of the really impor- 

 tant symmetry that of the arrangement of the materials 

 within the egg, which is no longer rotatory about the egg axis, 

 but bilateral with reference to the gravitational plane. 



The final development of the internal structure or organiza- 

 tion of the egg is completed (in Ranafusca) only shortly before 

 the first cleavage, or about an hour and a half after the 

 entrance of the sperm (Brachet). Before the end of the 

 first hour, the structure of the egg is gradually becoming 

 fixed and disturbances or artificial lesions are compensated, or 

 regulated, so that the final structure is not affected and later 

 development is not abnormal. But after this, by the time the 

 egg and sperm nuclei have fused, the egg structure becomes fixed 



