222 FINE-STRUCTURE OF PROTOPLASM II 



If the nucleoli represent reserve proteins, their formation is com- 

 parable to that of aleurone grains in the cytoplasm. In fact, it has been 

 observed that the protein crystalloids which sometimes replace the 

 nucleoli grow in small vacuoles of the nucleus. The place where the 

 nucleoli appear is predetermined, for they condense in contact with 

 special chromosomes provided with secondary constrictions (Heitz, 

 1935; Hakansson and Levan, 1942). At first they behave like real 

 vacuoles, for in the presence of several chromosomes condensing 

 nucleolar material, the several nucleoli formed can subsequently unite 

 to form bigger ones. In the present state of our knowledge the 

 nucleolus formation must be considered as an accumulation of the 

 karyolymph proteins at a definite spot, which takes place at the ex- 

 pense of energy, until a coacervate droplet rich in proteins is formed. 



Nuclear spindle. The microscopic structure of the spindle which 

 becomes apparent in nuclear divisions has long remained an enigma. 

 In fixed preparations spindle-shaped fibrillae are visible, some of which 

 stretch from the one pole of the cell to its equator, while others, 

 shorter ones, coalesce with the chromosomes at special points of 

 attachment (centromeres). In the living state, however, all this re- 

 mains invisible; microscopically the spindles are homogeneous, 

 structureless and optically empty. Microsurgical interventions reveal 

 a relatively rigid double cone with distinct cleavability but without 

 a visible structure (Belar, 1929). Accordingly the spindle fibres have 

 been considered as artefacts of the fixing process. 



In this case it has been possible to elucidate the true state of affairs 

 by means of the polarizing microscope. Schmidt (1937a) finds the 

 spindles to be positively birefringent in living sea-urchin eggs. Thus 

 the images visible in the fixed material prove to be real structures 

 existing in vivo. Since the poles of the spindle behave like positive 

 spherites whose rays can be followed nearly throughout the cell, they 

 must consist of optically positive invisible fibrillae. Undoubtedly 

 the same fibrils stretch from each pole to the chromosomes. It was 

 thought that these fibrils were submicroscopic and ought, therefore, 

 to be visible in the electron microscope. This is the case when acid 

 fixation is used (e.g. Bouin's solution; Beams, Evans, Verne van 

 Bremen and Baker, 1950). But, when duly fixed with neutral formahn, 

 the spindle region of dividing cells in onion root dps appears to be 

 structureless (RozsA and Wyckoff, 1950). Therefore, the fibrillar 



