84 DIFFERENTIATION III 



into three similar zones : a greenish- white layer at the centri- 

 petal pole, a middle clear stratum, and the blue yolk-granules. 

 These eggs develop normally even when continuously centri- 

 fuged. 



o 



In the Rotifer Hydatina (Whitney) the polar zones are 

 pink and grey, the middle clear as in the foregoing instances. 

 The stratification may have any relation to the original axis ; 

 the first cleavage is, as in the normal egg, transverse to this 

 axis, and normal young are produced, become mature, and 

 reproduce in their turn. 



Lastly, in the centrifuged egg of Ascaris similar strata 

 appear. The normal egg, as pointed out already, is telo- 

 lecithal. There are in the cytoplasm also some clear spherules 

 and pigment granules. The layers that appear after centri- 

 fuging are a layer of yolk at the centripetal end (the yolk 

 is here lighter than the cytoplasm), a layer of clear spherules, 

 protoplasm, and finalty, at the centrifugal pole, a cap of brown 

 pigment. When strongly centrifuged, the egg becomes flat- 

 tened against the slide on which it is placed (the centrifugal 

 force is perpendicular to the slide), and, if still subjected to 

 the action of the force, the fertilization spindle places itself 

 at right angles to the direction of the force, that is, parallel 

 to the stratification (and in the clear zone) and the division 

 is meridional. If, on the other hand, the egg is removed from 

 the machine it resumes its spherical shape and the spindle 

 returns, more or less completely, to its proper axial position 

 and the first division is equatorial (or oblique). It is sug- 

 gested by Boveri and Miss Hogue (to whom the experiments 

 are due) that there is an invisible polarized structure in the 

 cytoplasm which is not affected by the operation, and with 

 the axis of which the stratification of the movable substances 

 can make any angle. Into the axis of this invisible polarity 

 the spindle is supposed to return, if and when the egg is 

 allowed to resume its spherical shape. The facts do not 

 appear to necessitate this view, for when placed on the 

 machine the whole egg rotates inside its shell until the 

 heavier animal pole is centrifugal and then the stratification 

 of the cytoplasmic materials begins. As long as the force is 



