3o8 Transactions British Mycological Society. 



Fungi where "no differentiation of male or female exists in 

 some of the most important and, indeed, primitive types." As 

 Professor Hartog points out the term sex "originally implied a 

 binary differentiation of pairing cells into categories of distinct 

 size and habit." But the more we know of the physiology of 

 sex the more we see that this difference in size and habit is only 

 a morphological indication of profound internal differences. 

 Such differences may exist in fusing cells which are morpho- 

 logically identical, and there is no good reason why they should 

 not be regarded as sexual. Among the Mucors for example, as 

 Blakeslee has shown, isogamy is only morphological. " Sexually 

 the two (morphologically identical) gametes which unite have 

 diametrically opposite characters." 



The idea of sex may be thus extended, and quite justifiably 

 so, I think, to cell fusions which take place between cells of the 

 same size if they result in the production of a zygote, and are 

 characterised by similar phenomena to that of binary fusions. 

 Such cells although not morphologically heterogamous are phy- 

 siologically heterogamous, and the difference between male and 

 female is thus at bottom a physiological one and not morpho- 

 logical. 



Sexual fusion or fertilisation involves not only the fusion of 

 two cells, but also the fusion of their nuclei. This latter fact was 

 not definitely established until 1875 when O. Hertwig and 

 Hermann Fol independently discovered the fusion of the egg 

 nucleus with the sperm nucleus in the e^^ of the sea urchin. 

 These observations were soon confirmed and extended both in 

 animals and plants, but it was not until 1889 that any satis- 

 factory indication of the fusion of a male with a female nucleus 

 was observed in the Fungi, and it was not until 1896 that it was 

 definitely established. 



At that time the existence of sexual organs in the Fungi was 

 well known. Oogonia and antheridia had been seen in many 

 forms of the Peronosporeae, and the formation of zygospores 

 by the fusion of two equivalent cells, or two unequal cells had 

 been definitely observed in the Mucorineae. There were indica- 

 tions of sexuality in the Ascomycetes, and already it had been 

 surmised that the aecidium of the Uredineae might be the seat 

 of sexual organs. The passage of the protoplasm of a male cell 

 into a female cell had been clearly observed in Pythium de 

 Baryamim by Marshall Ward and de Bary. No fusion of nuclei 

 had however been seen. This is not surprising for at that date 

 it was not known whether the majority of the Fungi possessed 

 true nuclei. Strasburger (1884) had observed the presence of 

 true nuclei in Trichia fallax but, except for some deeply stain- 

 able granules which had been seen in various species of Fungi, 



