PHYSIOLOGY OF CELL-DIVISION 401 



cell-division seems, however, to be unusual without the presence 

 of definite centrosomes, so that the predominant role is prob- 

 ably to be attributed to the latter, the surface-change serving 

 mainly to initiate their activity. Probably conditions vary 

 considerably in different cells. It is interesting to note that 

 conclusions in many respects similar to the foregoing were ex- 

 pressed by Wilson fifteen years ago; after pointing out that 

 the influence of the aster on the cell-sm-face depends upon its 

 distance from the latter, he continues: "it is altogether probable 

 that a second factor lies in the physical changes taking place 

 in the peripheral protoplasmic layer at the time of division, but 

 this factor is itself probably dependent on the position of the 

 aster, since we know that the direction of the cleavage-plane 

 varies with that position. ""^^ 



To a large degree any such hypothesis as the above is tentative, 

 if not indeed chiefly valuable as a guide to research; it should, 

 hotv'ever, be noted that any theory of cell-division must of neces- 

 sity involve the synthesis of a large number of data from differ- 

 ent fields; further, that in endeavoring to form valid conceptions 

 of vital processes the formation of such syntheses is indispensable. 

 The dividing cell, like an}^ other li\dng system, has a unity which 

 cannot be experimentally analyzed without the destruction of 

 those vital properties whose elucidation is the very object of 

 biological research; our only hope of understanding these prop- 

 erties lies in a reconstruction based on all known relevant facts 

 and principles. Hence such reconstructions must be attempted, 

 not only for guidance in research, but in the interest of complete 

 knowledge. 



SUMMARY 



1. Sea-water of a dilution sufficient to cytolyze all unfertilized 

 Arbacia eggs in half an hour or less, causes osmotic swelling but 

 not cytolysis in uncleaved fertilized eggs (up to a few minutes 

 before cleavage begins). At or about the time of formation 

 of the cleavage-furrow a marked decline takes place in the re- 

 sistance of the egg to hypotony, and cj^tolysis is then rap d and 

 complete. When the cleavage-furrow is fully formed the origi- 



" Loc. cit., p. 377. 



