290 Action of the Genetic Material 



whether the genie material remains unchanged during all phases of 

 development, all cells remaining genetically omnipotent. If this is so, 

 we must explain why the action of the different parts of the genetic 

 material in controlling definite steps of development takes place only 

 at definite times and in definite cells. We must explain what has been 

 called the "activation" of individual genes at definite times and places. 

 (The word "activation" includes the idea that genes may remain 

 dormant until they start to function at a certain moment. I use the 

 word here without such a connotation, simply as describing the fact of 

 the orderly timing of genically controlled developmental steps.) The 

 alternative question is whether the genie material itself changes first 

 so as to restrain its genie qualities, select some and discard others by 

 actual loss or inactivation, or changes in whatever other way can be 

 imagined. Though this would not be exactly Weismann's erhungleiche 

 Teilung, it would amount to the same thing, because very frequently 

 development uses a single division to produce a specialized from a 

 generalized cell. In all meristematic growth, typical for plants, but 

 found also in animals in the teloblasts of worms, in the production of 

 nurse cells in the ovary, and in other cases, such differential divisions 

 occur: one cell retains all the genetic potencies and the other one is 

 determined to a single fate. The possibility of purely cytoplasmic 

 unequal distribution cannot be denied for this special division. We 

 have discussed the example of the unequal division of the sperma- 

 tocyte of aphids, in which the smaller cell lacking mitochondria has 

 a fate different from that of its larger sister cell. However, this would 

 not prevent the occurrence of intranuclear change in one product. 



Cytoplasmic influence upon chromosomal behavior is well known 

 since Boveri's work on chromatin diminution. Here the location of 

 the nucleus in one or another kind of cytoplasm, visibly differentiated 

 during oogenesis, determines whether or not the chromosomes will 

 undergo diminution. Probably the same principle is at work when in 

 the "segmenting" insect egg a nucleus, entering the posterior, visibly 

 differentiated germ-track cytoplasm, becomes a sex-cell nucleus and 

 henceforth behaves differently, in regard to rhythm of division, for 

 example, from the other nuclei. It is the same thing if, according to 

 Sonneborn (see 1955), the nucleus derived from the last meiotic 

 (equational) division in Paramecium becomes a fertilization nucleus 

 only when it becomes located in a definite part of the oral cytoplasm. 

 (Attention is drawn again to Lwoff's findings — see I 1 — that certain 

 self-reproducing organelles assume a definite pattern, according to 

 the part of the cytoplasm in which they are located. Though the 



