III. i GRAVITATION 87 



embryo. The grey crescent which appears soon after fertiliza- 

 tion and is regarded by Roux as a direct effect of this process, is 

 supposed by Moszcowski to be produced by the action of gravity 

 upon the egg-contents during the short interval before the 

 perivitelline space is formed and the egg able to turn over, 

 to be comparable, in fact, to the grey area or white plate 

 described by Born in his forcibly inverted eggs. Every normal 

 egg, therefore, has a ' gravitation ' plane of symmetry which 

 later on becomes, as in inverted eggs, the median plane of the 

 embryo ; nor are the eggs on the rotatory apparatus exempt, for 

 it is held that the work of gravity can be accomplished on them 

 even in the few moments before they are placed on the machine. 



With regard to the latter point both Kathariner and Morgan 

 have demonstrated that eggs kept in a state of perpetual 

 rotation in all directions, from the very moment of insemination 

 develop into perfectly normal, bilaterally symmetrical embryos, 

 while Roux has replied to the first part of the criticism by 

 pointing out that the grey area observed by Moszcowski was 

 not the normal grey crescent produced by the entering sperma- 

 tozoon, but the ( white plate ' of Born due to the incomplete 

 rearrangement of yolk and cytoplasm in an egg which had been 

 quite unintentionally prevented from assuming its normal 

 position. The grey crescent, indeed, Roux argues, could not 

 possibly be due to gravity in a normal egg, for it does not 

 appear until some time after the axis has become vertical. 



There seems, therefore, to be little room for doubt that Roux's 

 original contention, that gravity does not determine the sym- 

 metry of the egg and embryo in the Frog, is correct, although 

 it remains a result of considerable importance that this external 

 factor may be made artificially to induce a bilaterality in the 

 egg which is sufficiently strong to persist as the symmetry of the 

 embryo. 



There is one other matter of interest in this connexion. It is 

 obvious, and has been experimentally shown by O. Hertwig, 

 that a centrifugal force can replace gravity. On a wheel rotated 

 with sufficient velocity the eggs turn with their axes radial, their 

 white poles outermost. If the velocity is great enough (145 revo- 

 lutions a minute, radius from 24 to 32 cm.) the yolk is driven 



