310 Transactions British Mycological Society. 



definitely established ip the first ten years of this period, that 

 is between 1889 and 1900. 



Thus in 1889 the chief stages of nuclear behaviour in the 

 maturation of the zygote of Peronospora were described. It was 

 shown that the oogonium contained over 100 nuclei, each of 

 which divided at least once, so that the oogonium contained 

 some 200 nuclei or more, that the fully formed oosphere con- 

 tained only one nucleus, that a branching tube from the anthe- 

 ridium carrying one nucleus penetrated the oosphere, that sub- 

 sequently an empty antheridial tube was seen, that the oosphere 

 then was found to contain two nuclei presumably male and 

 female, and that at a later stage only one nucleus was visible. 

 The conclusion was arrived at therefore that a male nucleus 

 passes over from the antheridial tube into the oosphere, and 

 finally fuses with the central nucleus. That this is what actually 

 takes place was definitely proved in 1896 for Cystopus candidus, 

 and in 1900 for Peronospora parasitica, and has been abundantly 

 confirmed by many observers. Here, then, we have a definite 

 sexuality, viz. the fusion of morphologically differentiated male 

 and female organs. The female organ is a large egg cell, contain- 

 ing abundance of cytoplasm, the male organ is a smaller cell 

 containing several nuclei and protoplasm, but only one of the 

 nuclei, with probably no cytoplasm, or only a very minute 

 quantity, migrates from the male organ into the egg cell. Mor- 

 phologically then this sexual fusion differs in no essential respect 

 from what takes place in higher plants and animals. But this 

 well-marked sexuality is not maintained throughout the other 

 groups of Fungi, and subsequent investigations show that pro- 

 found modifications of the sexual process occur. The first in- 

 dication of this was discovered in 1891 (Report of the British 

 Association, 1891) in the Hymenomycetes, in which no sexual 

 differentiation had so far been observed. It was found that two 

 nuclei were present in the young basidium, and that these two 

 nuclei fused together before the formation of the basidiospores. 

 The structure of the nuclei was found to be similar to that of 

 the nuclei in the higher plants : each "consists of a nuclear mem- 

 brane enclosing a dense nucleolus and a thread-like network." 

 In a later paper, 1893, the fusion of the nuclei was described in 

 detail, and the subsequent division of the fusion nucleus was 

 also described; a spindle figure, chromosomes and centrosomes 

 were observed, and the extrusion of the nucleolus into the cyto- 

 plasm. The number of chromosomes could not be determined 

 exactly, but all the figures published show eight or ten chromo- 

 somes in the division of the fusion nucleus, and four or five in 

 the daughter nuclei. In later papers it was shown that the 

 number of chromosomes in the vegetative nuclei was four, in 



