442 BREAKDOWN OF GENETIC SYSTEMS 



thenogenesis (e.g., Chara crinita, Ernst, 1918 ; Cocconeis placentida, 

 Geitler, 1927) and haploid parthenogenesis [e.g., many fungi, 

 Camarophylltis, Bauch, 1926 ; Schizophyllum, Kniep, 1928 ; 

 Saprolegnia, Mackel, 1928). 



3. NON-RECURRENT APOMIXIS 



The omission of one of the two essential sexual processes (meiosis 

 and fertilisation) without the other is necessarily a non-recurrent 

 abnormality of the life cycle, for it would otherwise lead to 

 indefinite reduction or indefinite multiplication of the numbers of 

 the chromosomes. Such an omission can therefore be expected only 

 as a result of exceptional conditions, whether genetic or external, 

 whether natural or artificial — if the distinction can be made. 

 These kinds of conditions have often been found. Of the three types 

 of result, one, the formation of unreduced gametes, especially in 

 hybrids, is of great importance in the origin of polyploids. It 

 has been considered in relation to the breakdown of meiosis (Ch. X). 



(i) Parthenogenesis. This abnormality, the omission of fertilisa- 

 tion without the omission of meiosis, has, with a few doubtful 

 exceptions, been observed to give mature progeny only in certain 

 flowering plants of various groups. The early stages of develop- 

 ment of this kind have been found in some animals and in Cutleria 

 (Phseophyceae) by Yamanouchi (1912), where the female gameto- 

 phyte was protected from fertilisation. The egg-cell develops with- 

 out fertilisation into the embryo sporophyte. The development has 

 been observed at all stages in Solanum (j0rgensen, 1928) ; in other 

 cases it is inferred from the seedling being of a reduced size and 

 having the reduced chromosome number and external character of 

 one parent. Table 66 records the evidence of this kind in the 

 best authenticated examples. In the last group, the seedlings 

 resembled their male parent and had half its chromosome number, 

 so that the inference was drawn that a male nucleus entered the 

 egg-cell, whose nucleus died while the pollen nucleus gave rise to the 

 embryo. This " male parthenogenesis " — or, it might be said, 

 cuckoo-parthenogenesis — is analogous to that stimulated artificially 

 in Vaucheria (v. Wettstein, 1920), and in echinoderms, where it is 



