ROBERT CHAMBERS. 



lifted, so that there is plenty of room within the membrane, per- 

 mitting the two blastomeres to assume almost spherical shapes 

 (Fig. 20-c). When the cleavage furrow is completed the two 

 blastomeres are contiguous only where the two spheres touch. 

 At this place the hyaline plasma layers of the two blastomeres 

 merge. We have here, apparently, two opposing forces ; -first, 

 the jellied aster holding each blastomere to a spherical shape, 

 and, second, the affinity of the plasma layer substance surround- 

 ing the two blastomeres. As soon as the asters disappear and 

 the cytoplasm of the blastomeres reverts to a more fluid state 

 the plasma layers of the two blastomeres merge more and more 

 and the blastomeres are pulled together till they assume shapes 

 approaching those of hemispheres (Fig. 2O-r). The outlines in 

 Fig. 20 are camera lucida drawings taken during the successive 

 stages of one sand-dollar egg. 1 



In the starfish, where there is no appreciable hyaline layer, and 

 where the fertilization membrane is lifted far beyond the surface 



i It has recently been intimated that the microdissection method is unre- 

 liable as a means of ascertaining changes in viscosity in the dividing egg 

 because of supposed discrepancies in the results obtained by Seifriz ('20) 

 and myself C'i7 b and '19). As a matter of fact the results of Seifriz har- 

 monize perfectly with mine. Seifriz states " there is a pronounced decrease 

 in viscosity of the central region of the cell with the first appearance of 

 the amphiasters." This statement has been interpreted as running counter 

 to mine. This is not true for although my results indicate that the astral 

 portion of the amphiaster is jellied, I definitely state (p. 494, >: 7) that the 

 central region and the zone between the two halves of the egg are fluid 

 where " a distinct flow of granufes medianward can be observed." 



Again, on completion of cleavage Seifriz notes that the two blastomeres 

 become liquid. This statement also fits in with my results. I state (p. 51. 

 '19) that, immediately after cleavage and while the two blastomeres are 

 still spherical, the firmness of the cytoplasm persists. Later, when the asters 

 disappear the cytoplasm liquefies and the two blastomeres crowd up against 

 one another. Seifriz noted this last liquid state of the two blastomeres 

 without considering the state prior to it. 



I may mention here a possible criticism of the centrifuge method in ascer- 

 taining viscosity variations. There are critical stages in the developing asters 

 during which agitation causes their disappearance. This was noted long ago 

 by Wilson. On bringing the eggs to rest the asters reappear and develop- 

 ment proceeds normally. I have already discussed this matter fully ('19). 

 The centrifuge and miscrodissection methods of studying the physical state 

 of protoplasm should serve as valuable checks on one another, if only the 

 investigators in these fields would agree on cooperation. 



