466 BREAKDOWN OF GENETIC SYSTEMS 



unfertilised egg has been induced to develop in many Echinoderma 

 and Algae, in the frog and elsewhere. Where the stimulus is applied 

 after the two polar bodies have been extruded from the animal egg 

 a haploid embryo develops. In the frog, where this has happened, 

 somatic doubling has followed later as in the somatic origin of 

 polyploid plants and in some kinds of diploid parthenogenesis. The 

 general failure of a diploid organism produced in this way to reach 

 maturity is probably due to the presence in most outbred organisms 

 of factors lethal in the homozygous condition. The same result is 

 obtained when a sperm fertilises an enucleated egg (" male 

 parthenogenesis "). Where reduction has not taken place the 

 stimulus may lead to the suppression of one of the meiotic divisions 

 or to the fusion of the products of the second meiotic division (as in 

 the production of diploid spores of Funaria by v. Wettstein, 1924). 

 An embryo may then develop without fertilisation. Such individuals 

 do not usually get beyond an early stage of development. 



It is possible to induce the direct development of an unfertilised 

 gamete in many Algae where meiosis should immediately follow 

 fertilisation, in various ways. Thus when the antheridia or oogonia 

 of Vaucheria hamata (v. Wettstein, 1920) were pricked they developed 

 directly into new thalli : both fertilisation and meiosis were omitted. 

 Apospory was induced by layering the fronds in Nephrodium (Digby, 

 1905). This automatically led to apogamy. 



The development of adventitious embryos from the nucellus or 

 integument has been induced by wounding in (Enothera Lamarckiana 

 by Haberlandt (1921 a). He also found that degeneration of the 

 nucellar tissue and synergids was associated with parthenogenesis 

 in Taraxacum and Hieracium, while in sexual species of Hieracium 

 no such degeneration occurred (1921 h). These observations make 

 it plausible that degeneration of neighbouring tissue may stimulate 

 apospory and parthenogenesis, and thus be a prior, and genetically 

 determined, condition of these phenomena. 



(iii) Mutation. Paula Hertwig (1920) found a sexual strain of 

 Rhahditis pellis which gave rise to a purely parthenogenetic strain 

 breeding true to the new character. It had the same chromosome 

 number as the parent race, but its egg-cells underwent a single 

 division at meiosis and gave unreduced eggs. The entrance 



