MITOCHONDRIA IN TISSUE CULTURES 387 



never shown that fat droplets arise from mitochondria (fig. 

 24 b, c, d). A thread may form a loop, but the loop changes 

 back again into a thread instead of continuing into a ring. 

 Various rings studied have never changed into globules during 

 observation but have become rods or threads or granules. Such 

 appearances as figure 24 e, were caused by the migration of a 

 mitochondrium to the edge of a vacuole and not as both Dubreuil 

 and Guilliermond might conclude, that the mitochondrium 

 formed the vacuole. Certain granules or thick rods seen in the 

 living cell have the appearance of hollow bodies in the perma- 

 nent preparations and correspond to some of Dubreuil's figures. 

 This appearance may be due to fixation as Kingsbury ('11) 

 suggests, i.e., that the osmic acid reduced more at the surface 

 and later the more soluble interior is dissolved out. Both 

 Meves ('08) and Duesberg ('11) describe the clear inner part of 

 the mitochondrium, to quote Duesberg, the mitochondria were 

 first present in the early rabbit embryo as small granules but 

 these increase in volume and become large granules at the end 

 of the third day. They have a clear central part with a dark 

 outer edge. Such appearance was seldom seen in the living 

 cell and it is possible that these as well as certain figures of 

 Dubreuil and Guilliermond w^ere formed by the method of fix- 

 ation. Cells which contain both loop and ring shaped mito- 

 chondria frequently show no sign of fat formation, while other 

 cells which are accumulating fat show no mitochondria of the 

 shape which Dubreuil leads us to suppose form the fat droplets. 



There are three distinct types of fat in these tissue culture 

 growths. First, that in the cells which grow out from tissues 

 that at the time of explantation of the piece of tissue, con- 

 tained fat droplets as the yolk membrane or the migrating fat 

 cells. There seems to be a predetermined ability on the part 

 of these cells to form fat, as is clearly shown where the growth 

 from the yolk membrane adjoins that from the connective tissue 

 (fig. 25). Each new yolk membrane cell contains fat droplets 

 similar to those of the explanted piece of the yolk membrane. 

 In these cells the mitochondria are usually in the form of small 

 granules and the fat droplet is surrounded by granules which 



