Ill DIFFERENTIATION 85 



operating, the spindle is compelled to place itself parallel to 

 the stratification, but when released from the force, returns 

 or attempts to return to its normal position, namely, in the 

 egg (i. e. in the stratification) axis. The obliquity of the 

 spindle, in those cases where the return to the normal 

 position is not complete, would then be the result of two 

 tendencies at right angles to one another, the one urging the 

 spindle to place itself perpendicular, the other parallel, to the 

 stratification. 



When the spindle returns more or less completely to its 

 normal situation, the division is equatorial or oblique and 

 a normal embryo is developed in spite of the stratification. 

 When, however, the spindle remains in the stratification plane, 

 the first division is meridional and each cell behaves as the 

 P l (vegetative) cell of an entire ovum. The greater part of 

 the pigment zone is usually extruded from these eggs at 

 the centrifugal pole, as a ' ball'. Each half-blastomere divides 

 into two, which can be recognized as E M St and P 2 by the 

 chromosomes being diminished in the one and intact in the 

 other, and by their subsequent behaviour, and so gives rise 

 to .what is essentially a blastula without ectoderm (see Fig. 5, 

 p. 13). It might be imagined that the ectodermal material 

 had been extruded with the ' ball ', but apparently this is not 

 so since the development is the same when (as may happen) 

 no ball is extruded. 



It must be admitted that it is at the moment very difficult 

 to build any very definite hypothesis upon the results of 

 these various experiments with the centrifuge. It is obvious 

 that by the rearrangement of some at least of the constituents 

 of the cytoplasm a stratified polar structure may be easily 

 imposed upon the egg, and that it is certainly not necessary 

 that all of these materials should return to their original 

 positions in order that development may be normal. The 

 evidence does, however, indicate here and there (the Frog, the 

 Sea-urchin, the Pulmonate) that time must be allowed for 

 a recovery of some kind before development can be normal. 

 The axis of the new polarity may entirely replace the original 

 axis in the determination of embryonic symmetry (as in the 



