APOMIXIS AND RELATED PHENOMENA 411 



In some cases^^ it is reported that two maturation divisions occur 

 without accompHshing haplosis. More commonly there is but one 

 division, a single polar body being formed with no haplosis. ^^ In 

 Rhabditis monohystera, a nematode worm, the 20 chromosomes in the 

 oocyte form 10 synaptic pairs and separate in the single meiotic mitosis 

 which occurs. During the telophase the chromosomes appear double as 

 in ordinary meiosis, but since there is no second mitosis to separate them 

 the egg retains the diploid number, 20. This egg develops without 

 syngamy into a diploid animal. Apparently in rare cases there may be no 

 maturation divisions. In Neuroterus lenticularis, a gall-fly, the nucleus in 

 certain eggs approaches the surface as if to produce a polar body but then 

 returns to the middle of the egg and divides equationally. Development 

 then proceeds with the unreduced chromosome number, 20 (Doncaster, 

 1910, 1911, 1916). 



Parthenogamy. — Under this heading may be placed those compara- 

 tively rare cases in which an egg develops without union with a male 

 gamete but after a fusion of two nuclei. For example, it has been 

 reported that a polar body nucleus may sometimes fuse with the egg 

 nucleus, after which development proceeds." In Mactra, after the forma- 

 tion of the two polar bodies has been induced by treatment with salt 

 solutions, the nucleus of the egg divides into two which t-hen fuse, and 

 development proceeds (Kostanecki, 1911). The fusion of cleavage 

 nuclei at later stages has been observed in certain species of Solenobia 

 (Seller, 1923). 



False Hybrids. — As in plants, so in animals there are rare instances 

 of the development of an embryo which, though supposedly the offspring 

 of two parents of the same or different species, contains the functional 

 nuclear factors of only one of them. Cases like that of Nereis (p. 242), 

 in which the egg can be activated by a sperm without the latter's entrance, 

 may be included here. Those in w^hich entrance occurs may be grouped 

 under the following heads. 



A. Gynogenesis. — Here the male nucleus degenerates in the cyto- 

 plasm of the egg, development proceeding with maternal nuclei alone. 

 This appears to occur normally in Rhabditis, a genus of nematodes. The 

 eggs form only one polar body and retain the unreduced chromosome 

 number. They are then penetrated by the sperm, which degenerates 



"^^ Rhodites rosce (Henking, 1892; Schleip, 19096; Hogben, 1920a), Nematus lacteus 

 (Doncaster, 19076). 



■^^ Phylloxera (Morgan, 1906), Aphis (de Baehr, 1908-1912, 1920a), Daphnia 

 (Kiihn, 1908), Miastor (Kahle, 1908), Simocephalus (Chambers, 1913), Lecanium 

 (Thomsen, 1927), Rhabditis (Belaf, 1923), Aspidiotus (Schrader, 1929), probably 

 Tettigidae (Robertson, 1930). 



25 Artemia (Brauer, 1894; F. Gross, 1932), Asterias (O. Hertwig, 1890; Buchner, 

 1911). 



