2l8 BIOLOGICAL LECTURES. 



From our new point of view we may briefly reexamine the 

 rate of cell-division in the cleavage-blastomeres. 



Let us take, in the first place, as a specific example, one of 

 the prototroch-cells of Amphitrite (Diagram I). It would be 

 difficult to maintain that this cell is unable to divide because of 

 the absence of a vigorous centrosome, inasmuch as it must 

 inherit this structure as an heirloom from the previous cells in 

 whose mitosis there was no indication of waning activity. The 

 inhibition of the division of the prototroch-cell would seem not 

 to depend upon its position, nor upon the absence of an organ 

 of division, but upon the metabolic activity peculiar to the cell 

 by virtue of its internal structure. 



The blastomeres adjacent to the prototroch-cells have a dif- 

 ferent structure, and consequently a different metabolic activ- 

 ity, one expression of which is a difference in their rate of 

 cleavage. 



We would not imply that the rate of cleavage of the blasto- 

 meres is unaffected by stimuli from without the cell, coming 

 from intercellular secretions, from the medium in which the 

 egg lies, or from some other source ; indeed, the effect of 

 potassium chloride upon the oocyte and egg-cell is evidence to 

 the contrary. The position occupied by one blastomere may 

 be more favorable for the reception of these stimuli than that 

 occupied by another ; yet the extreme differences in the rate 

 of division exhibited by cells which are adjacent in time or in 

 space are out of all proportion to the differences in their 

 positions. 



The peculiar organization of the cell determines the charac- 

 ter of its response to a stimulus determines whether the cell 

 shall or shall not divide. 



