458 BREAKDOWN OF GENETIC SYSTEMS 



in their meiosis is conditioned by autopolyploidy which prevents 

 regular reduction as effectively as does structural hybridity. 



(iii) Meiosis in Parthenogenetic Haploids. The occurrence of 

 meiosis in a mother-cell is affected in an entirely different way by 

 haploidy according as the fertilisation which has failed is normally 

 premeiotic or post-meiotic. The first type has been found in algae 

 and fungi, the second in the angiosperms (Ch. X), and in animals 

 with haplo-diploid sex differentiation. 



In Vaucheria hamaia (v. Wettstein, 1920) it is possible to stimulate 

 both male and female gametes to development without fertilisation 

 or the meiosis which would normally follow it. Parthenogenesis 

 can be similarly induced (by treatment with dilute solutions) in 

 Sptrogyra, Ulothrix, and Saprolegnia mixta (Mackel, 1928). 



In the sexual basidiomycetes the conjugating nuclei fuse in the 

 basidial cell and immediately undergo meiosis. The four daughter- 

 nuclei then pass into the four terminal sacs, where they develop into 

 spores. But in some species only two sacs are produced, and in 

 these it is believed that two nuclei pass into each (Sass, 1929). In 

 parthenogenetic species the single nucleus of the basidial cell divides 

 once where two sacs are developed (Camarophyllus virgineus, Bauch, 

 1926, V. Fig. 133), and twice where four sacs are developed 

 (Schizophyllum commune, Kniep, 1928, p. 422). These observations 

 are paralleled in the Uredinese (cf. Rosenberg, 1930). 



Thus, in forms where fertilisation immediately precedes meiosis, 

 the omission of fertilisation means, sometimes, the suppression of 

 meiosis, e.g., in the parthenogenetic race of Camarophyllus virgineus, 

 Bauch (1926) found early prophase stages reminiscent of meiosis, 

 although the later course of the single division in the basidium was 

 purely mitotic (n — 4). Elsewhere there appears to be a complete 

 absence of any meiotic process, and the nucleus instead divides 

 mitotically twice, once or not at all as purely vegetative conditions 

 dictate {Vaucheria, v. Wettstein, 1920). Habitual parthenogenesis 

 can therefore be induced (artificially or by mutation) at one step in 

 such an organism as Chara crinita, while in diploid organisms the 

 assumption of parthenogenetic habit must necessarily be more 

 complicated, since it means a change in a specialised meiotic 

 mechanism. 



