Ill DIFFERENTIATION 83 



With the egg-axis, as so determined, the axis of stratifica- 

 tion may make any angle. Fertilization occurs, and in the 

 subsequent cleavage the planes of division bear the normal 

 relation to the axis of the egg. The strata persist, so that 

 the red pigment may be in AB or CD and so on, the green 

 oil on the opposite side. Development follows and these two 

 coloured materials are found opposite to one another in any 

 position in the trochophore larva, the structure of which has 

 the normal relation to the cleavage system. There does, 

 however, appear to be some tendency for the green oil to 

 redistribute itself. 



So again in Pulmonate eggs (Physa, Planorbis, Limnaea) 

 Conklin has, by the same means, produced three strata : a 

 grey finely granular zone at the centripetal pole, a narrower 

 clear zone, and a yellow granular centrifugal hemisphere. 

 When segmentation and development take place the strata 

 make any angle with regard to the first and subsequent 

 furrows, and any angle with the principal planes of the 

 embryo. Conklin has, however, added the important observa- 

 tion that the possibility of obtaining a normal development 

 is largely dependent on the redistribution of some of the 

 disturbed cytoplasmic materials, for it is only when the 

 operation is performed prior to maturation or during its 

 earlier stages only, that is, when some time elapses between 

 the operation and cleavage that development is afterwards 

 normal. Eggs centrifuged during the extrusion of the first 

 polar body, or later, either die or give rise to monstrous 

 embryos. It appears further that during the interval, the 

 clear substance disappears into the grey or the yellow layer 

 or both, a readjustment which cannot occur unless sufficient 

 time be allowed. In another Mollusc, Crepidula, on the 

 other hand, and in the Ascidian Cynthia, Conklin has found 

 it possible, by prolonged centrifuging, to shift the original 

 polar axis (which in the experiments just quoted is left 

 unaltered) without prejudice to normal development. The 

 symmetry of cleavage and of differentiation are, it seems, 

 determined by the new polarity as in the Frog's egg. 

 v . In Cyclops (Spooner) the centrifuge separates the cytoplasm 



G2 



