III. i GRAVITATION 85 



is anterior (more correctly, anterodorsal), for it is here that the 

 dorsal lip of the blastopore appears ; the opposite end is posterior 

 (more correctly, postero-ventral). 



The pigment moves with the cytoplasm ; it is, however, 

 unable to completely displace that yolk which remains at the 

 upper surface in consequence of the greater viscosity of the 

 superficial rind, and here a ' white plate ' or ' grey patch ' is 

 formed. Similarly, at the lower surface the pigment is not 

 necessarily wholly displaced by the descending yolk. 



There is one special case that may be noticed. When the inver- 

 sion is complete (180) the yolk flows radially and peripherally 

 away from the upper pole while the cytoplasm ascends in the axis. 



Bern's observations make it perfectly clear that gravity re- 

 arranges the contents of these inverted eggs, and so confers 

 upon them a secondary structure like that of the normal, and 

 symmetrical about a secondary axis which is likewise vertical. 

 To this secondary axis the direction of elongation of the karyo- 

 kinetic spindles, and consequently the cleavage planes, bears the 

 same relation as in the normal egg ; and there is certainly no 

 more need to explain these directions by reference to gravity, 

 to suppose, in fact, a causal connexion between the two, in the 

 one case than in the other. The planes, indeed, may fall where 

 they do simply because the mitotic figures elongate in the direc- 

 tion of least resistance (Pfliiger) or (O. Hertwig) in that of the 

 greatest protoplasmic mass, or may be related, in some similar 

 way, to the structure of the egg alone. 



The point can only be determined by an experiment in which 

 the directive influence of gravity is eliminated. This experi- 

 ment has been made by Roux. The eggs were fastened in 

 small vessels, at distances of from one to eight centimetres 

 from the centre, to a wheel rotating continually about a hori- 

 zontal axis, but so slowly (one revolution in from one to two 

 minutes) that the centrifugal force developed was insufficient to 

 make the eggs turn with the white pole outwards, and therefore 

 negligible. The direction of the force exerted by gravity upon 

 them from moment to moment was thus not constant. Of the 

 eggs some were free to rotate inside their jelly, others were 

 fixed. To anticipate the objection that the plane of rotation, 



