THE CYTOPLASM 215 



known, and it is therefore not possible to determine the extent of mitochondrial 

 contamination that could account for the enzymes supposedh* associated with mela- 

 nin granules. Furthermore, the suggestion that melanin granules are modified forms 

 of mitochondria and that some pigmented cells do not contain unpigmented mito- 

 chondria'" has not received support from more recent studies with the phase contrast 

 and electron microscopes." 



The isolation of chloroplasts by differential centrifugation was first reported by 

 Granick.'^ It is of interest that the isolation of these structures was most successful 

 when sucrose solutions were used as the media. 



(J. Isolation of Secretory Granules 



The isolation of secretory granules from liver'^ and pancreas^'* ■'* was reported by 

 Claude. The identification of the granules from liver as secretory in nature was dis- 

 puted by Lazarow'5 and Hoerr," who felt that the isolated structures were mito- 

 chondria. Although Claude'* later admitted that a large proportion of the granules 

 was probably mitochondria, he maintained that at least some were secretory granules. 

 Subsequently, Hogeboom et a/.'^ found that most of the secretory granules of the 

 liver cell apparentl}- did not exist as formed elements when the cells were disrupted. 

 They also found that the isolated mitochondrial fraction was free of secretorj' gran- 

 ules and that cytoplasmic elements staining with neutral red (e.g., secretory granules 

 and lipid droplets) migrated cent ripet ally when homogenates prepared in sucrose 

 solutions were centrifuged. It would appear from these observations that the isolation 

 of secretory granules in pure form remains to be accomplished. 



h. Isolation of Golgi Apparatus 



The history of the Golgi apparatus has been fraught with uncertainty because of 

 difficulties in its cytological identification in fresh preparations. A part of the failure 

 to demonstrate the structure in the living cells of all tissues has been the lack of agree- 

 ment concerning its vital staining properties. On the one hand, it is claimed that the 

 Golgi substance or its precursor is stainable in unfixed cells with neutral red or meth- 

 ylene blue, while on the other, it is claimed that the material is refractory to staining 

 with these dyes. Recentlj', Worlej'" reported the identification of the Golgi appa- 

 ratus in the form of droplets that stained with neutral red and were found in the 

 soluble or nonsedimentable fraction of liver homogenates. Identification was based 

 upon the observation that the droplets formed artificial Golgi networks, similar to 

 those seen in fixed preparations of liver on addition of Nile Blue sulphate, but no 

 further attempts were made to separate or characterize the bodies. With regard to 

 the identification of these droplets as the Golgi apparatus on the basis of the neutral 

 red. stain, it is necessary to recall that Hogeboom et al.^^ also observed the presence 

 of droplets stainable with neutral red in the nonsedimentable fraction of liver. They 

 observed, furthermore, that there were two tj'pes of droplets with similar staining 

 properties in the intact unfixed liver cell: one, 0.5 to 1.0 /* in diameter, located at the 



" S. Granick, Am. J. Botany 25, 558 (1938). 



'* A. Claude, Cold Spring Harbor Symposia Quant. Biol. 9, 263 (1941). 



'» A. Claude, Biol Symposia 10, 111 (1943). 



'* A. Lazarow, Biol. Symposia 10, 9 (1943). 



" N. L. Hoerr, Biol. Symposia 10, 185 (1943). 



" A. Claude, J. Exptl. Med. 80, 19 (1944). 



" L. G. Worley, Exptl. Cell. Research 2, 684 (1951). 



