III. i GRAVITATION 81 



crescent. This side becomes the dorsal side of the embryo, while 

 the animal pole marks, approximately, the anterior end. 



By complete disappearance of the pigment the grey crescent 

 becomes added to the white vegetative area of the egg. 



The foregoing account applies in particular to liana temporaria 

 and R. fusca ; R. palitstris appears to be similar, but in R. escu- 

 lenta it is stated that the egg-axis is eventually not vertical but 

 oblique (Fig. 44). It seems, however, doubtful whether this 

 obliquity is not rather apparent than real. The grey crescent has 

 apparently not been recognized as such the pigment is not so 

 deep as in the other species but included, nevertheless, in the 

 white area, with the result that the centre of this, the definitive 



FIG. 44. Egg of Eana esculenta after fertilization, in its normal 

 position with the axis oblique (?). A, from the side ; 5, from above ; 

 a a, egg-axis ; mm, plane of first furrow. (After Korschelt and Heider.) 



white area, has been confounded with the centre of the original 

 unpigmented area or vegetative pole of the vertical egg-axis. 

 (Compare Fig. 44 A with Fig. 43 B.) 



As is well known, the planes of division during the first few 

 regular phases of segmentation bear a perfectly definite relation 

 to the axis. The first two, at right angles to one another, are 

 meridional and therefore also vertical, the third furrows are parallel 

 to the equator, therefore also horizontal; the furrows of the 

 fourth phase are again meridional, and hence vertical, those of 

 the fifth once more latitudinal and horizontal. It is this obvious 

 relation of the planes of cleavage to the direction of gravity 

 which has raised the question whether there is not a causal con- 

 nexion between the two, the question which Pfliiger attempted 



JENKINSON 



