THE BIOLOGY OF THE CELL SURFACE 



of other material — would take place at that stage in the 

 cleavage-process when the blastomeres on separation lose 

 the potency for development into complete though dwarf 

 larvae. But is an egg-cell (in this case with determinate 

 cleavage) JBCD = AB + CD, as we would express it 

 according to the theory of segregation ? Or can we say that 

 AB = AB + i-CD); CD = CD + {-AB) ? Can we say 

 that ABC Dh really AB + {-CD) + CD + {-AB) and do 

 we gain thereby ? 



Restriction is, as I shall show, preferable to segregation 

 for it more nearly expresses the process. Restriction 

 implies a loss, in this case of developmental potencies, 

 without necessarily a rearrangement of materials. Segre- 

 gation connotes new arrangement or sorting out of mate- 

 rials without any implication of change in them. If as 

 cleavage progresses cytoplasmic materials for embryo- 

 formation are merely shifted to new positions, then reversal 

 of the cleavage process should be possible. Stages of dif- 

 ferentiation then would be repetitive. I do not wish to 

 indulge in hair-splitting definitions; nevertheless I must say 

 that I for my part can not see how the materials of an egg by 

 mere acts of separation can account for differentiation. 

 Against the assumption of a mere transposed order of 

 materials stands the fact that as development proceeds the 

 individual blastomeres lose potency. 



At first cleavage blastomere AB becomes such because 

 it loses CD-material, and blastomere CD becomes such 

 because it loses .^5-material. A separation into AB- 

 and CZ)-blastomeres means that AB somehow removes 

 all CZ)-material and CD all .-/^-material. So does gene- 

 tic restriction begin and so it continues. This takes 

 place through the absorption of CD-material by the 

 nucleus of the ^^^-blastomere-to-be and of the ^5-material 

 by the nucleus of the CD-blastomere-to-be. The subtrac- 



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