APOGAMY, APOSPORY, AND PARTHENOGENESIS 317 



(6) Without meiosis: the gamctophyte is developed upon the sporo- 

 phyte by budding; that is, spore-reproduction is replaced by a 

 vegetative process: for instance, in mosses it has been found 

 possible to induce the development of protonema, the first stage 

 of the gametophyte, from tissue cells of the sporogonium: 

 [In this way fil. and Em. Marchal (1909, 1912) were able to 

 produce in Mnium, Bryum, Phascum, and Amblystegium 

 diploid gametophytes; these in turn produced tetraploid 

 sporophytes which bore diploid spores. In one case (Ambly- 

 stegium) a tetraploid gametophyte was regenerated from 

 cells of the tetraploid sporophyte.] Similarly, in certain ferns 

 (varieties of Athyrium Filix-fcemina, Scolopendrium vulgare, 

 Lastrcea pseudo-mas, Polystichum angulare, and in the species 

 Pteris aquilina and Asplenium dimorphum), the gametophyte 

 (prothallium) is developed by budding of the leaf of the sporo- 

 phyte [commonly from the margin of the leaf or from the tissue 

 of the sorus (Fig. 126)], and in some of these cases it has been 

 ascertained that the gametophyte so developed has the same 

 number (2x) of chromosomes in its nuclei as the sporophyte 

 that bears it that is, it is diploid. 



Apospory has been found to be associated frequently with 

 apogamy [in the life cycle]; in fact, in the absence of meiosis, 

 this association would appear to be inevitable." 



PARTHENOGENESIS IN ANIMALS' 



The natural development of an egg without having been fertilized by 

 a male gamete is a phenomenon which is apparently of much more 

 frequent occurrence in animals than in plants. The best known examples 

 are found among the rotifers, crustaceans, and insects, parthenogenesis 

 being the regular mode of reproduction in some species. Other modes 

 also usually occur in such organisms under certain conditions or after a 

 certain number of generations. Parthenogenesis is reported in some 

 protozoa (Plasmodium vivax, Schaudinn 1902), where the macrogamete, 

 after certain nuclear changes, continues the life cycle without fusing 

 with a microgamete. Moreover, as has already been described in the 

 preceding chapter, parthenogenesis may be artificially induced in the 

 eggs of other animal groups, notably echinoderms, mollusks, and amphi- 

 bians, and around this fact centers much of the significant work of modern 

 experimental biology. In commenting upon parthenogenetic develop- 

 ment Minchin (1912, p. 137) points out that " . . . the gamete which has 

 this power is always the female; but this limitation receives an explanation 

 from the extreme reduction of the body of the male gamete and its 



1 The cytological results of researches on maturation and development in cases of 

 parthenogenesis have recently been summarized by Paula Hertwig (1920). 



