122 



CLEAVAGE AND DIFFERENTIATION 



of the materials which it contains, for it may contain only oil and 

 cytoplasm, or these on one side and yolk on the other. It was further 

 found that on centrifuging the egg at the time of the first cleavage, 

 the nuclear spindle may be disarranged so as to divide the egg 

 equatorially with reference to the original axis. In such cases the lobe 

 appears with reference to the original axis, i.e. at the end opposite 

 the polar bodies, and not with reference to the cleavage plane.^ 



Fig. 58 



Effects of centrifuging the uncleaved eggs of the Mollusc Ilyatiassa. A (modified) 

 polar lobe is formed at the first cleavage ; it forms at the region opposite to the 

 polar bodies — i.e. in its normal relation to the original egg-axis — irrespective of 

 the direction of the first cleavage furrow or of the materials it contains. (From 

 Morgan, youni. Exp. Zool. lxiv, 1933.) 



It appears that during mitosis, most of the egg undergoes some 

 degree of gelation, but that the region of the polar lobe is not 

 involved in this, and that the superficial layer of the lobe-region 

 is predetermined during the oocyte stage to behave as it does 

 during maturation and early cleavage. It is interesting to note that 

 if the polar lobe is detached before the first cleavage, it undergoes 

 spontaneous changes of form, apparently synchronised with the 

 cleavages of the egg.^ 



• §6 



For mosaic development to occur, some degree of precocious 

 chemo-difTerentiation must have been effected, prior to the onset 

 of cleavage. The organ-forming materials thus available may be 



1 Morgan, 1933. 



