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 











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 

 Maternal x 

 t Paternal x 

 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-workers shows that the two X-chromo- 

 somes are alike (XX) in the male-producers 

 but differ genetically (XX') in the female 

 producers. The results of fertilization in 

 these two types are diagrammed in Figure 

 49. 



The unusual chromosome constitution of 

 the zygote in Sciara results from peculiari- 

 ties in spermatogenesis. In the first sperma- 

 tocyte a monocentric spindle is formed and 

 the chromosomes are separated without pair- 

 ing, in such a way that all the paternal 

 chromosomes except the "limited" chromo- 

 somes are extruded in a small cytoplasmic 



go to one pole and are incorporated in the 

 single spermatid that forms, while the 

 chromosomes at the other pole are included 

 in a small bud that later degenerates. All 

 of the spermatozoa that form are, then, alike 



Female 

 ( 9-producing) 



Female 

 (rf-produclng) 



Germ line: 

 Gametes: 



( 9-producing) 



Fig. 49. Metz's interpretation of the production 

 of "unisexual" families in Sciara coprophila on the 

 basis of genetically different (X and X') sex chro- 

 mosomes (from Metz, '38). 



and contain a haploid set of autosomes, the 

 "limited" chromosomes and two X-chromo- 

 somes. Oogenesis, on the other hand, is regu- 

 lar with random segregation of chromo- 

 somes. 



Chromosome Elimination in Other Animals. 

 Even more extensive elimination of chromo- 

 somes from somatic cells has been described 



