IC)4 FINE-STRUCTURE OF PROTOPLASM II 



may accumulate in the vacuoles of storage ceils, where they crystallize 

 or solidify into aleurone grains. However, if the amount of high mole- 

 cular weight protein chains in the cytoplasm increases and these chains 

 cluster together, protoplasmic fibrils are formed (Kuster, 1934a, 

 1935 a). In other words, the morphological properties observed depend 

 upon whether reserve proteins or structural proteins are separated. 

 Originally the aleurone grains are liquid vacuoles, which lose water 

 by active dehydration. In this process the various vacuole components 

 precipitate according to their solubility. In the aleurone vacuole of 

 Rkinus seed, for instance, the almost insoluble magnesium-potassium 

 salt of inositol phosphoric acid (phytin) is precipitated first as a body 

 called "globoid". Thereupon the reserve proteins which, in contrast 

 to the insoluble skeletal proteins, are corpuscularly dispersed, begin 

 to arrange themselves into the lattice order of a crystalloid (cf. p. 136) 

 and to fill the available space. Finally the last remnants of liquid, 

 containing an easily soluble albumin, solidify into a homogeneous 

 substance surrounding both globoid and crystalloid. On mobilization 

 of the reserve substances, the dissolution proceeds in the reverse 

 order: the albumin is dissolved first, thereupon follows the protein 

 crystalloid and finally the mineral globoid. 



Origin of fibrils. Formerly the formation of contractile fibrils (Proto- 

 zoa) and of muscular fibres (Metazoa) was regarded as an extremely 

 curious achievement of the cytoplasm. Nowadays, however, this kind 

 of differentiation can be understood from a morphological point of 

 view, since the framework structure of the cytoplasm itself consists 

 of submicroscopic strands. These structural elements need only be 

 accumulated and arranged in some order to produce microscopic 

 fibrillar structures. However, the mechanism of contraction of these 

 fibrils remains obscure (cf. p. 359). 



Phase separation by centrifitging. The phases brought about by sepa- 

 ration can be stratified in the cell by centrifugal force. Here the 

 centrifuge microscope of E. N. Harvey and Loomis (1930) renders 

 special service. Fig. 113 shows a centrifuged sea-urchin egg of A.rbacia 

 punctulata. Centrifuging has elongated the egg cell and its various 

 components: pigment grains, yolk globules, mitochondria and oil 

 droplets appear neatly separated. Optically homogeneous cytoplasm, 

 containing the nucleus, accumulates in the less dense part of the cell. 

 The striking layer formation seems to indicate a stratification phe- 



