234 



vironmcnt, they can again function 

 normallv, unchanged in their funda- 

 mental characteristics by having been 

 associated with the wrong kind of 

 gene. Hence, plastids seem to have a 

 degree of independence of the gene. 



Plastid inheritance is therefore a 

 well known phenomenon and has been 

 studied intensivclv in a number of or- 

 ganisms. Since plastids or their pre- 

 cursors are usually transmitted through 

 the egg but not the sperm, their inheri- 

 tance is usually maternal in character, 

 i.e., the offspring receives its plastids 

 onlv from the mother. There are ex- 

 ceptions, however, where plastids may 

 also be introduced into the offspring 

 along with the sperm. But it is not 

 only the plastids which are of interest 

 from the standpoint of cvtoplasmic in- 

 heritance. The work of Sonneborn and 

 others has drawn attention to the fact 

 that certain hereditary characteristics 

 require the interaction of both the genes 

 and certain elements in the cytoplasm. 

 Tlie "killer" characteristic in Parame- 

 cium, a one-celled animal, is a case in 

 point. Some individuals in Paramecium 

 aurelia are capable of excreting a sub- 

 stance into the surrounding water which 

 will kill other "sensitive" animals. To 

 be a "killer" a cell must have a certain 

 gene (K) and in the cytoplasm it must 

 have minute particles which are known 

 as kappa particles. If both are present, 

 the animal is a killer. If, however, the 

 K gene is present and the kappa par- 

 ticles absent, the cell cannot produce 

 the killer substance. Kappa particles 

 are self-reproducing bodies, but they 

 can live and reproduce only in the 

 presence of the K gene. In the absence 

 of this gene they soon vanish. The K 

 gene, on the other hand, cannot syn- 

 thesize new kappa particles. If none of 

 the particles are present in a cell pos- 

 sessing K, none can be formed. In 

 other words, kappa particles must come 

 from preexisting kappa particles. Tlie 

 conditions under which these particles 



CYTOLOGY 



can divide and function are set up by 

 the K gene, but the K gene cannot 

 create new particles. The particles are 

 therefore dependent upon the gene 

 for the proper environment in which 

 to continue existence, but not for their 

 origin; the gene is dependent on the 

 particles to carry out the process of 

 killer substance formation. Tlius a 

 Mendclian or hcreditarv character is 

 dependent for its expression on both a 

 gene and a t^■pe of cytoplasmic par- 

 ticle. A number of other cases of this 

 type of inheritance are known. 



Another t\pe of particle in the cyto- 

 plasm is the chondriosome, or mito- 

 chondrion. Chondriosomcs have been 

 known for 50 years or more but they 

 have not received much attention until 

 recentlv. They are ordinarily minute 

 spherical or rod shaped bodies, about 

 the size of bacteria, with which they 

 have been confused by some investi- 

 gators. For a long time their function 

 remained obscure, although their uni- 

 versal presence in all plant and animal 

 cells indicated that they had a vital 

 role to plav. In recent years, however, 

 a combined biochemical and c\tolog- 

 ical approach has shown that the\' are 

 the chief centers of respiration, the 

 regions where most of the enzvmcs are 

 situated which together bring about the 

 liberation of the energy needed by the 

 protoplasm. This discover\' has added 

 a new importance to the chondrio- 

 somcs and has focused renewed atten- 

 tion upon them. 



The renewed interest in the cyto- 

 plasm has resulted in an increased use 

 of the electron microscope in order to 

 learn more about the structure of the 

 bodies found in this region. Since these 

 bodies are so small their structure is 

 beyond the capabilities of the ordinary 

 microscope to elucidate, and electron 

 microscopy seems the only feasible way 

 now available to get at the details of 

 their architecture. The results of re- 

 cent studies by such workers as Frey- 



