. Analysis of development. Embryology; Embryology. Gametogenesis, Fertilization and Parthenogenesis 173 cleus in both male and female larvae. This elimination is described (Berry, '41) as oc- curring in a resting nucleus, the chromo- some being expelled through the nuclear membrane into the cytoplasm. Presumably there is also an elimination of one or more "limited" chromosomes from the germ cells bud while the maternal chromosomes and the "limited" chromosomes all move to the pole and are incorporated in the single sec- ond spermatocyte. The latter forms a bi- polar spindle
. Analysis of development. Embryology; Embryology. Gametogenesis, Fertilization and Parthenogenesis 173 cleus in both male and female larvae. This elimination is described (Berry, '41) as oc- curring in a resting nucleus, the chromo- some being expelled through the nuclear membrane into the cytoplasm. Presumably there is also an elimination of one or more "limited" chromosomes from the germ cells bud while the maternal chromosomes and the "limited" chromosomes all move to the pole and are incorporated in the single sec- ond spermatocyte. The latter forms a bi- polar spindle, and the sister chromatids of each chromosome except the X separate to opposite poles. Both chromatids of the X Egg 0 Early Cleavoge II Sperm Zygote '--, Somatic >SV^ rS Line , Ste *>-? . * ^.... Spermatogonia Is-t Division Selective Segregation 2nd Div 2nd Elimination ^^. _ Duriiuno [B] ?^^ ^'^ ^m^ <^MU S\V^ <,v^ 'a q: Soma sperm I Limited Cliromosomes 0 Maternal x t Paternal x 0 Autosomes Fig. 48. Diagram of behavior of the chromosomes in Sciara coprophila, illusti'ating difference between so- matic and germ line cells in regard to elimination of chromosomes. The female germ line (not illustrated) would be the same as in diagram A except that the meiotic divisions do not involve the cytological pecu- larities exhibited in spermatogenesis. See text for further description. (From Metz, '38.) in some cases, but this has not, as yet, been described. Since the germ cells of Sciara have the same chromosome set in the male as in the female the determination of whether they are to produce spermatozoa or eggs is evi- dently effected by the somatic cells which, as a result of the elimination of one or two X's at the seventh (or eighth) division, have the typical XO constitution in the male and XX in the female. This, in turn, is evidently determined in the female parent which, as a rule, produces unisexual families. The genetic evidence accumulated by Metz and his co-worke
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