34 The Structure of Protoplasm 



The evidence presented so far shows a close correlation between 

 the behavior of protoplasm and the properties of proteins. It has 

 been shown that protoplasm can behave like a thixotropic, plastic 

 gel composed of elongated protein molecules arranged like a disor- 

 ganized brush-heap, except when in certain active states. Obviously, 

 not all protoplasm is the same; the lability of its framework permits 

 it to exist in extremely liquid forms. 



BEHAVIOR OF SEA URCHIN EGG PROTEINS AND MYOSIN 



Properties of the Proteins. With the exception of myosin, the 

 proteins we have discussed above have not been shown to take part 

 in protoplasmic structure. It is, therefore, especially interesting 

 that undenatured proteins involved in protoplasmic structure have 

 been isolated by Mirsky (24) from the eggs of Arhacia and Strongylo- 

 centrotus. The eggs were frozen at — 77°C., dried in vacuo, ground, 

 and then extracted with cold 1 M KCl at pH 7.3. Mirsky found that 

 about 83 per cent of the total protein in the egg could be dissolved 

 by this method. If, however, the eggs had been fertilized, the 

 soluble fraction would have been reduced to approximately 70 per 

 cent. In other words, about 15 per cent of the soluble protein is 

 rendered insoluble by the fertilization process. 



Mirsky could show that the change in this protein fraction 

 (which may not be a single protein) was not associated with cell 

 division or the elevation of the fertilization membrane. The change 

 in solubility occurred in an interval of 3 to 10 minutes after fertiliza- 

 tion, with no further change detectable in the next 2 hours. It is, 

 however, in this interval that the first cleavage occurs. Mirsky 

 found that this decrease in solubility was accompanied by an 

 increased strength and elasticity of the egg. The processes of freez- 

 ing and thawing broke unfertilized eggs but did not break fertilized 

 eggs frozen in this time interval. The fertilization membrane was 

 not responsible for this phenomenon. He concludes that a skeleton 

 framework, capable of supporting the cytoplasm during develop- 

 ment, is produced by the alteration of this protein.-' 



To test these results, Moore and Miller (29) investigated the 

 appearance of the Strongylocentrotus egg between crossed nicols. 

 Its optical behavior was somewhat obscured by the cytoplasmic 

 granules. After these had been moved to one side of the unfertilized 

 egg by centrifugation, it was found that the hyaline cytoplasm was 

 isotropic. If, however, the eggs were fertilized, the hyaloplasm 

 became clearly birefringent within 3 minutes. This likewise sug- 

 gests an orientation of structural elements. 



