MEIOSIS 277 



synapsis is influenced by a considerable group of factors, including gene 

 homology, a tendency to form pairs, and peculiar conditions at chromo- 

 some ends. It is successful and wholly "normal" only when the actions 

 of the various factors are so correlated as to permit homologies to be 

 completely satisfied. When the normal relationship of the several 

 influences is disturbed, as in structurally unbalanced types especially, a 

 variety of unusual results may be expected. 



Not only do these phenomena bear upon our hypotheses of synaptic 

 attraction, but they raise fundamental questions regarding the role they 

 probably play in the production of translocations, deficiencies, and other 

 modifications of the chromosome complement of significance in the 

 alteration of species. They further serve to emphasize the importance of 

 a knowledge of the finer morphology of the individual chromosomes in the 

 interpretation of the phenomena of meiosis. Much that has been written 

 about meiotic chromosome configurations must be reevaluated in the 

 light of critical evidence of this kind. 



Meiosis in Lower Organisms. — In the foregoing account of meiosis 

 only higher plants and animals have been considered. This section will 

 summarize briefly the conditions in lower groups, with special reference 

 to the stage in the life cycle at which meiosis occurs. The statements 

 should be compared with those made in the preceding two chapters 

 regarding the other reproductive phenomena in these groups. 



Among the Protozoa, ^^ the matter of chromosome organization and 

 behavior in many groups is very obscure. In others, however, definite 

 chromosome numbers and meiotic phenomena essentially like those in 

 higher organisms have been demonstrated. In most species with a 

 permanent union of gametes the vegetative stages have the zygotic 

 chromosome number, and haplosis occurs in connection with gameto- 

 genesis,^^ as in Metazoa (Fig. 161). In the infusoria, which have a tem- 

 porary union of cells known as conjugation (see p. 245), meiosis occurs in 

 the first two divisions of the micronucleus just prior to the formation of 

 gamete nuclei.''" In a few sporozoa the vegetative stages have nuclei 

 with the gametic number of chromosomes, meiosis taking place in the 

 zygote after the fusion of the sexual nuclei.^' In one of these, Aggregata 

 eberthi, the monoploid set is made up of one long, one short and four 

 medium-sized chromosomes. There is also genetic evidence that in the 

 phytoflagellate Chlamydomonas meiosis occurs in the zygote (Pascher, 



^ See Minchin (1912), Jennings (1920, 1929), Calkins (1926), and Belaf (1926). 



'' E.g., ActinosphcBrium eichhomii (R. Hertwig, 1898b) and Aciinophrys sol (Schau- 

 dinn, 1896; Belaf, 1922). The paper by Belaf is especially noteworthy. 



'^'^ Opercularia (Enriques, 1907, 1908), Chilodon (Enriques; MacDougall, 1925), 

 Carchesium (Popoff, 1908), Uroleptus (Calkins, 1919), Oxytrichia (Gregory, 1923), 

 Euplotes (Turner, 1930). For further cases, see Calkins (1926, p. 518). 



*^ Aggregata eberthi (Dobell and Jameson, 1915), Diplocystis schneideri (Jameson, 

 1920). 



