THE BIOLOGY OF THE CELL SURFACE 



pigment granules, each at a different rate, shift to new posi- 

 tions, always the same in every undisturbed egg of a given 

 species and definite at its every cleavage. In the order 

 and amplitude of this shifting we recognize the power of 

 the so little known cytoplasmic currents. Since these in 

 turn are directed by ectoplasmic changes, as I have shown, 

 we may by study of the shifting of the inclusions come to 

 know more about the ectoplasm and its interaction with 

 the inner cytoplasm. 



In this inner cytoplasm is situated that fixed and little 

 changing cell-component, the nucleus, to which has been 

 assigned the role of maintaining intact the inheritance of 

 the species. What is its role in differentiation? 



Along with the successive acts of cleavage by which the 

 egg substance is divided up into blastomeres and as the 

 embryo-axis or plane of symmetry becomes recognizable, 

 the original single nucleus of either the fertilized or the 

 parthenogenetic egg undergoes division so that each blasto- 

 mere receives a nucleus. In discussing the probable role 

 of the nucleus as a causal factor in the differentiation during 

 cleavage, I shall now deal with the following points: the 

 quantity of nuclear material finally found at the close of 

 the cleavage-period, the origin of this material, and how 

 the nuclei, with special reference both to chromosome- 

 content and to the genes making up the chromosomes, may 

 be conceived as taking part in determining the development 

 of the egg. 



Although with each successive nuclear division during 

 cleavage of the egg the nuclei progressively diminish in 

 size, no individual nucleus in any blastomere at the end of 

 cleavage ever vanishes. On the other hand, the total 

 quantity of nuclear substance, where this has been deter- 

 mined, is greater at the end than at the beginning of the 

 cleavage-period. The quantitative increase in nuclear 

 substance continues through the whole embryonic period 



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