The early cytologists came to the conclusion that in a majority of organisms the leptotene chromosomes were single and that this singleness persisted at least until pachytene. This, in turn, led to the idea that chromosome duplication occurred during pachytene and not, as in mitotic tissues, during interphase. True there were claims to the contrary. Some workers held the leptotene chromosomes to be visibly double (see Rhoades 1961) but these claims were either treated as suspect or else could be fairly simply explained. Thus, in Tradescantia and Trillium it is possible to resolve half chromatid units with an ordinary light microscope (Nebel and Rutile 1936) so that the chromosomes at leptotene do show a bipartite structure. But, since the chromosomes at first metaphase are quadripartite, the leptotene threads can still be regarded as single in the sense that the prophase chromosomes of mitosis are double.