The Life Cycle of the Single Cell 25 



1. A rhizoid containing a nucleus (med) was joined to an anucleate 

 segment of stalk ( cren ) . After a few months of incubation a new cap was 

 formed. Its appearance was intermediate between med and cren. If, how- 

 ever, the newly formed cap were cut off, a second cap was produced and 

 this was a med-type cap with no remnant of cren characteristics. Further- 

 more, additional removals of the cap yielded only med-type caps. The 

 reverse experiment ( cren rhizoid with nucleus and a med anucleate stalk 

 segment) also yielded at first a cap which was intermediate between the 

 cren and med types. When this cap was removed, all subsequently formed 

 caps were purely cren. 



2. Two nucleate rhizoids, one of each species, were joined, cut end 

 to cut end. At the junction a stalk grew out which formed a cap inter- 

 mediate between med and cren. Any subsequent removals of the cap 

 yielded regenerates still intermediate between the two species. 



3. Two nucleate rhizoids of cren were joined, cut end to cut end. 

 The cut end of a third nucleate rhizoid (med) was fused to the junction 

 point of the first two. A stalk grew out from this point whose cap was 

 intermediate in appearance but much more like cren then med. When the 

 experiment was reversed, i.e., the two med rhizoids joined with one cren 

 rhizoid, the cap was much more like med than cren. 



From these experiments we conclude that it is the nucleus which 

 ultimately controls the morphogenetic activities of Acetahularia, pre- 

 sumably by directing the synthesis of substances that play specific roles 

 in the construction of the cap. These materials are to some extent stored 

 within the anucleate portions of the stalk but are not spread equally 

 throughout. Thus, pieces of stalk taken from the apex have more of them 

 than pieces taken from the base; longer stalk pieces have more of them 

 than shorter pieces. 



With these experiments as a background, it now is possible to in- 

 quire into the nature of the biochemical reactions that mediate morpho- 

 genesis in Acetahularia and into how the nucleus exerts control over them. 

 Such problems represent an exciting challenge to young biologists and 

 may lead to a general understanding of morphogenetic events in many 

 organisms. 



