The cell physiology of early development 



1948). Perhaps the best investigated are the factors following clearly cytoplasmic 

 inheritance in crosses between different races of the willow herb, Epilobium. Another 

 example is provided by the cytoplasmic factors causing male sterility in crosses 

 between races of a number of different species of plants (e.g. flax, maize, etc.). 

 Evidence for such factors in higher animals is exceedingly rare. The case which 

 perhaps seems most likely to fall into the category is that recently described by Laven 

 (1953) in mosquitoes of the genus Culex. He shows that in crosses between certain 

 species there is a factor, transmitted cytoplasmically through the eggs, which causes 

 the sperm of the animals carrying it to be incompatible with the cytoplasm of the 

 eggs of certain other races. This may be a true plasmagene. On the other hand, its 

 sporadic occurrence in the races of Culex from certain localities, and the existence of 

 rather similar factors in certain other nearly-related species of mosquito, raises the 

 possibility that it may be rather an exogenous virus to which certain local races are 

 adapted, while in others it produces the symptoms of damage to the sperm. 



None of the entities in this category of true plasmagenes can yet be seen and there 

 is no direct evidence as to the size of particle involved. The indirect evidence, chiefly 

 from the type of physiological effect which they produce, is usually held to suggest that 

 they are bodies of a gene-like order of complexity. It is not impossible, however, that in 

 the future some of them may turn out to be simpler than has been previously thought. 



(3) Cytoplasmic particles with genetic continuity 



One should perhaps make a separate category for the comparatively large par- 

 ticles, visible with the microscope, which exist in the cytoplasm of some forms and 

 are endowed with a genetic continuity; that is to say, in which each particle is 

 normally, and perhaps always, derived from a similar previous particle. Examples 

 are the kinetosomes in ciliates, and chloroplasts in green plants. They seem to 

 differ from the true plasmagenes in the previous category not only in their larger 

 size but also in the complexity of their relation with the nuclear genes. The true 

 plasmagenes do not appear to be altered in their character by the nature of the 

 nuclear genes with which they are associated, although they do, of course, enter 

 into physiological relations with those genes during development. The character 

 of the cytoplasmic particles, on the other hand, seems to be often, and perhaps always, 

 under rather direct nuclear control. Very many genes are known in plants which 

 control the formation of chlorophyl and the chloroplasts; and Weisz (1951) has 

 described a most interesting set of interactions between the nucleus and the kine- 

 tosomes in the Protozoan Stentor, related to the differentiation of the regions of the 

 body. There are in this form reciprocal interactions between the nucleus and the 

 cytoplasmic particles. On the one hand the macronucleus controls the type of 

 organelle (cilium or mouth part, etc.) which the kinetosome can form during regener- 

 ation : on the other, the kinetosomes in the posterior region of the body influence 

 the nodes of the macronucleus which lie in that region, and deprive them of the 

 capacity to cause the differentiation of anterior organelles. 



(4) Gene-initiated plasmagenes 



In contrast to the preceding categories there are a group of factors, which are also 

 often considered to be plasmagenes, and which are characterized by the fact that 



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