The Cytoplasm as Specific Substrate 237 



mutant affects both chloroplast and, if the assumption is correct, 

 mitochondrial behavior. This might be used as an argument for the 

 mitochondrial origin of plastids (see Sonneborn, 1955), but it might 

 just as well mean a direct (mitochondria) and an indirect (plastids) 

 inhibitory effect upon respiratory activity of the cytoplasm, condi- 

 tioned by the respective mutants. 



We have spoken many times of cytoplasmic features under the 

 control of the nucleus. Since the genome is supposed to exercise its 

 controlling function upon the cytoplasm, which is the seat of all 

 morphogenetic and physiological processes in the cell, it is not sur- 

 prising to find specific differentiations of the cytoplasm like plastids 

 and mitochondria (whatever their nature and origin) under genie 

 control. We have also mentioned that features of the nucleus and its 

 chromosomes may, vice versa, be influenced by the cytoplasm. This 

 may look like primary cytoplasmic conditioning (i.e., cytoplasmic 

 heredity) which controls even the nucleus, but such a conclusion 

 would be wrong. If we take examples like Ascaris diminution or the 

 germ-track cells of insects, discussed earlier, there can be no doubt 

 that the differentiation of the ooplasm, which controls nuclear features, 

 has been brought about by processes taking place during oogenesis 

 under the influence of the nucleus. All the facts of predetermination 

 discussed previously prove this to be true. This does not exclude the 

 existence of cytoplasmic features which are not autonomous but rather 

 dependent upon the environment. If, as Schleip (1929) reports, the 

 polarity of the egg cell is sometimes determined by its primary 

 location in the ovarial epithelium, we have an example of a non- 

 autonomous determination, namely, conditioned by the environmental 

 one; a similar example is the relation of the symmetry plane in the 

 frog egg to the path of the sperm. These facts clearly have nothing to 

 do with cytoplasmic heredity via autonomous differentiations. 



A good example of this nucleo-cytoplasmic interrelation in both 

 directions is found in Sonneborn's work on mating types of Para- 

 mecium ( 1955 ) . He showed in very ingenious experiments that mating 

 type is controlled by the macronucleus. But under the influence of 

 specific cytoplasm the mating type can be changed (the active cyto- 

 plasm is introduced by bridge formation in conjugation). However, 

 the cytoplasmic action takes place only when newly formed macro- 

 nuclei are present (by division of the syncaryon). If, under specific 

 circumstances, a macronucleus in the exconjugant is formed from 

 rests of the old macronucleus, the cytoplasmic action in changing the 

 mating type fails. Further experiments by Nanney, Sonneborn, and 



