THE BIOLOGY OF THE CELL SURFACE 



in size; they may be at first equal in size and early in the 

 succeeding cleavages show marked size-differences or they 

 may from the first cleavage show striking disparity in size. 

 At the end of the cleavage-period the blastomeres are of 

 unequal size in all totally cleaving eggs. 



The position of the blastomeres also plays a part in 

 making the cleavage-pattern in totally cleaving eggs. If 

 they stand directly above each other the final pattern is 

 radial; if the four smaller cells, instead of lying directly 

 over the larger ones, lie above the furrows of the first and 

 second cleavage, the pattern is spiral. 



The derivation of the blastomeres may be so regular 

 that one can determine exactly in the cleavage stages the 

 time and place of the origin of each. For many eggs it 

 has been shown that the blastomeres in late cleavage can 

 be traced unerringly back to the stage of first division. 

 Thus, for these eggs, a cell-lineage, as it is called, can be 

 traced. In other eggs for several cleavages the blasto- 

 meres arise always in the same way and hence show con- 

 stancy in origin; after this period, they vary both as to 

 time and place of origin. In other words, the cleavage 

 pattern of animal eggs may be constant or not. If their 

 cell-lineage can be traced, the pattern so far as the origin 

 of the blastomeres is concerned, is constant. It is to be 

 noted that the cleavage of eggs of clams, worms, ascidians, 

 showing cell-lineage is closely similar. Hence this type of 

 cleavage is not restricted to any one group of animal eggs. 

 If the lineage of the cells can not be traced, this means that 

 the cleavage-pattern is composed of cells whose positions 

 are not determined by fixed origin. 



Partial cleavage is of two forms, discoidal and superficial. 

 In the former, cleavage is confined to a disc at only one pole 

 of the egg, whilst in the latter the entire superficial cyto- 

 plasm cleaves. Discoidal cleavage Is found in eggs of 

 cuttle-fishes and their allies, in many bon}' fishes and in 



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