A cytological demonstration of "genetic" crossing-over

A stage in meiosis where the paired chromosomes form loops, coming together at points distributed at random along their length, was first described by Rückert (1892) inPristiurus . These points we now recognise in suitable material as exchanges of partner amongst the four pairing half-chromosomes. But although Rückert did not appreciate this detail of structure he grasped the possible significance of their relationship: "Die bei der Reifung der Geschlechtzellen eintretende Verklebung Vorhergetrennter Chromosomen bezweckt einer Substanzaustausch derselben." He considered that here was the possibility of a chromosome "amphimixis," carrying a step farther Weismann's idea of nuclear "amphimixis." Similarly Correns (1902) and Boveri (1904, Contrary to his earlier view) foresaw the possibility of different parts of a chromosome behaving independently at meiosis. The discovery of linkage by Bateson, Saunders and Punnett in 1905 was a verification of these predictions, although not at first interpreted in this sense.