84 EXTERNAL FACTORS III. i 



forcibly inverted eggs the cleavage planes had the normal relation 

 to the vertical, but not to the egg-axis ; he observed, however, 

 that the first furrow was usually in or at right angles to the 

 streaming meridian, the plane, that is, including the original and 

 the secondary vertical axes. The recent examination of 215 

 cases by the writer has shown that the first furrow tends to lie 

 either in, or at right angles to, or at an angle of 45 to this 

 plane. Subsequent development was normal, and the sagittal 

 plane coincided with the streaming meridian. The dorsal lip 

 appeared on the white side, which is thus anterior (anterodorsal). 

 The examination of sections, however, showed that in these 



FIG. 45. Sections through forcibly inverted Frog's eggs. In A the egg 

 has just been inverted, in B the streaming of protoplasm upwards and 

 yolk downwards has begun. Both sections are in the streaming meridian 

 or gravitation symmetry plane, including both the axis (a a) and the 

 vertical. bD, pigrnented animal protoplasm ; wD, unpigmented vegeta- 

 tive yolk, a, animal pole ; a, vegetative pole ; i, pigment-free clear 

 area ; k, nucleus ; p, superficial pigment. (After Born, from Korschelt 

 and Heider.) 



inverted eggs there had been a redistribution of the contents, 

 the heavy yolk sinking to the lower side, the lighter protoplasm, 

 with the pigment and the nucleus, and the spermatozoon rising 

 to the upper side (Fig. 45). The movement is rotatory, the cyto- 

 plasm and yolk ascending and descending in opposite directions ; 

 and it also takes place naturally parallel to, and in a similar 

 manner on each side of, the plane in which the primary and 

 secondary axes lie, hence known as the streaming meridian. That 

 end of this plane towards which the protoplasm moves in its 

 ascent, the end, that is, marked by the primary vegetative pole, 



