(1) Meiosis is described in three diploid and one aneuploid species of Calceolaria. The diploids are C. clibrani (2n=18), C. dentata (2n=18) and C. Banksii (2n=18). The aneuploid species, C. mexicana, has 2n=60.
(2) In the diploids, one bivalent is attached to the nucleolus at diplotene and diakinesis, while two bivalents are attached to the nucleolus at zygotene and diakinesis in C. mexicana.
(3) At diplotene in the diploids, interstitial chiasmata persist in 35% of the cases but are terminalised at metaphase. Multivalents are rare. In the aneuploid species associations of chromosomes ranging from trivalents to groups of eight are described. Rings and chains of four and six chromosomes were also found.
(4) An analysis of secondary association in the diploids at Metaphase I and II shows a maximum grouping of 3 (2)+1 (3). The diploid number 2n=18 is, therefore, considered to have been derived from the basic number 4, indicating allopolyploidy.
(5) In the diploids the presence of inverted segments is indicated by the formation of chromatin bridges at Anaphase I.
(6) The nature and significance of ring formations in the aneuploid species, the value of structural changes in evolution and the origin of the aneuploid species have been discussed.