Protoplasmic Streaming — Relation to Gel Structure 139 



sol ^ gel reactions play an important role in the mechanism which 

 operates to cleave the cytosoma.^'' This view has also been supported 

 recently by other types of work (Schechtman, '37; Chambers, '38; 

 and Dan and co-workers, '37) . 

 (1) The plas7nagel systeyn of egg cells 



On the basis of a number of recent observations, it is possible to 

 compare the egg cell with the Amoeba and to distinguish two well 

 differentiated parts of the cytoplasm: (1) the plasmagel,-" a corti- 

 cally situated layer,-^ about 5 microns thick, which displays marked 

 gelation changes during the different phases of division, and (2) the 

 plasmasoh the more fluid internal cytoplasm, which participates in 

 the protoplasmic streaming at the time of cleavage, and in which 

 the spindle and the asters are formed. 



The existence of a differentiated cortical plasmagel layer in the 

 Arbacia egg was first demonstrated clearly by the centrifuge experi- 

 ments of Brown, '34c, and of Costello, '34. In these experiments it 

 was observed that the formed elements of the cortex undergo cen- 

 trifugal displacement much less easily than those in the medullary 

 region of the egg, and that displacement of the cortical pigment 

 requires higher centrifugal forces, and longer centrifugal periods. 

 And Costello further observed that the apparent viscosity of the 

 internal protoplasm increases markedly as the temperature falls, 

 whereas the cortical layer, as judged by the displacement of the 

 pigment granules, becomes slightly less "viscous" in the range 

 between 20° and 2° C. 



Subsequently there have been a number of confirmatory observa- 

 tions. Motomura, '35, and Schechtman, '37, noted that the protoplas- 

 mic currents of the frog's egg, which appear at the time of the 

 cleavage furrow, involve only the deeper cytoplasm, and that visible 

 granules in the peripheral gelled layer are not disturbed by the 

 stream. Chambers, '38, determined that the thickness of the plas- 

 magel layer in the furrow region of the Arbacia egg is about 5 

 microns, by the method of watching the furrow impinge upon a mass 

 of oil which had been injected into the stalk between the blasto- 

 meres, replacing the plasmasol in this region. Furthermore, Cham- 

 bers has observed-'- that when one of the two potential blastomeres 

 of a dividing egg is punctured, practically all of the internal cyto- 

 plasm pours out from the unpunctured blastomere, through the 

 aperture surrounded by the impinging furrow, leaving a rind of 

 protoplasm which is especially thick bordering the furrow. This 

 cortical remnant displays the properties of a gel, not only in that it 



