80 PROTOPLASM OF PROTOZOA 



Relative specifc gravity of cell inclusions and components. — It has 

 long been known that the various inclusions of protozoan cells are of 

 different specific gravities. In centrifuging cultures to obtain large num- 

 bers of organisms for fixation previous to morphological studies, it is 

 often noticed that certain crystals have been displaced centrifugally. Mc- 

 Clendon (1909) was one of the earliest workers to fix and stain Parame- 

 cium after long-continued centrifuging; he found that the crystals and 

 nucleus were displaced centrifugally. Heilbrunn (1928) mentions that 

 a centrifuged Euglena loses its spindle-shaped contour and becomes 

 spherical, with the granular inclusions packed at the centrifugal end. 

 The same author (Heilbrunn, 1929b) has used the speed of movements 

 of crystals centrifugally through the cytoplasm of A. dubia to estimate 

 the absolute viscosity of the protoplasm; the specific gravity of the 

 crystals was estimated to be approximately 1.10. E. N. Harvey (1931) 

 records that the crystals of A. dubia fall down so rapidly that their 

 velocity can hardly be determined in the microscope-centrifuge; and 

 that the crystals of Paramecium were rapidly thrown down, as was the 

 nucleus. He was also able to cleave living Stentor into two parts in the 

 microscope-centrifuge; the lighter, oral half contained none of the 

 Zoochlorellae which had been moved into the basal part. E. N. Harvey 

 and Marsland (1932) observed the movement of cytoplasmic particles 

 through the protoplasm of A. dubia and found them to be layered out in 

 the following order: coarse granules and crystals, most centrifugal; nu- 

 cleus, a visibly empty zone, a zone of fine granules, and, most centripetal, 

 the contractile vacuole. Mast and Doyle (1935b) have recorded as 

 follows the relative specific gravities of the various cytoplasmic com- 

 ponents in A. proteus, from centrifugal to centripetal: refractive bodies, 

 beta granules (mitochondria) and food vacuoles containing little or no 

 fat, nucleus and food vacuoles containing much fat, hyaline protoplasm, 

 contractile vacuole, crystal vacuoles without crystals, and fat globules. 

 The position of the crystal vacuoles varies with the size of the included 

 crystals: those with large crystals are heavy and move centrifugally in 

 the centrifuge; those with small crystals are lighter. The small alpha 

 granules, which are about 0.25 micron in diameter, are not layered out. 

 All the refractive bodies, a large proportion of the crystals, and all the 

 fat may be removed, with no injurious effects, from a centrifuged 

 amoeba by cutting off the light and heavy ends. However, removal of 



