58 S. S. COHEN 



mast cells^ and pancreas to form the secretory granules^ and indeed represents 

 a general pattern of development of secreted morphological structures. 

 Kecent developments on secretory granules are summarized by Schneider 

 and Hogeboom (1956). 



In addition to tliis type of transformation, considerable discussion has 

 arisen over the possible mitochondrial origin of the chloroplasts. One school 

 holds that there are two kinds of mitochondria, one of which gives rise to 

 pjastids and fails to stain with Janus green B (Newcomer, 1951). Other 

 students of this question beheve that plastids arise only from plastids. 



Some workers consider that mitochondria divide and multiply independ- 

 ently of cell division, as do chloroplasts and some intracellular parasites. As 

 will be discussed below, Lindberg and Ernster (1954) have concluded that 

 the microsomal fraction of cytoplasm is converted into mitochondria, while 

 Ephrussi (1953) considers it likely that mitochondria demonstrate a con- 

 siderable measure of genetic continuity requiring self-duplication. These 

 opinions are not necessarily contradictory, since the latter hypothesis may 

 be thought of as referring to the continuing multiphcation of fuUy formed 

 mitochondria. Nevertheless, the effects of carcinogenic azodyes and acriflavin, 

 to be mentioned below, are usually taken to imply that the genetic continuity 

 of mitochondria is not continually restocked from other cellular fractions. 

 On the other hand, an instance is recorded in which egg fragments of the sea 

 urchin, apparently deprived of mitochondria, were induced to develop into 

 larvae whose cells were restocked with this cellular particle (Harvey, 1946). 

 Although this appears to bolster the view that mitochondria may be syn- 

 thesized de novo, these experiments were done at a time when electron 

 microscopy was not being used, and it was not proved conclusively that the 

 initial cells were devoid of all mitochondria. 



Mitochondria isolated in sucrose have been observed to possess a slight 

 constriction, which in time proceeds to an actual division in vitro into two 

 fragments (Leon and Cook, 1956). Whether this process bears any relation 

 to the in vivo increase of these elements is not clear. Novikoff (1956) has 

 recorded the presence in rat Mver homogenate of smaU mitochondria or 

 "mitosomes," possessing a greater number of electron-dense granules; it has 

 been suggested by Leon and Cook that such a group of mitochondria may 

 arise in a division process. However, such a class of small mitochondria is 

 now beheved to be biochemically unique, containing the "lysosomes." 



1 Contrary to the views of Zollinger (1950b), heparin does not appear to be associated 

 with the large granules of mast cells (Hedbom and Snellman, 1955); these large granules 

 possess all the chemical characteristics of mitochondria. They also contain histamine. 



2 Indeed, the so-called "secretory granules" of liver appear to dissolve upon homo- 

 genization (Hogeboom et al, 1948), a result entirely consistent with the above hypo- 

 thesis concerning their origin. 



