6 H EWITT, Cytological A spect of Parthenogenesis in Insects. 



cannot be said to be compensated for by thelyotoky, and, 

 as Sharp has pointed out, it is in the most abundant 

 species in this country, the currant saw-fly, Nematus 

 ribesii, that one finds arrhenotoky occurring. 



Doncaster (34) has recently investigated the matura- 

 tion and early development of the parthenogenetic ova of 

 several species of Nematus and allied forms. 



Nematus ribesii. Doncaster finds that the nuclear 

 changes in the maturation of the parthenogenetic ovum 

 of this species are very similar to those observed by 

 Petrunkevvitsch in the drone-egg. There are two successive 

 mitotic divisions of the nucleus {Figs. 1-4) which take 

 place in a line at right angles to the surface of the Qgg. 

 The outer half of the first polar nucleus becomes flattened 

 against the Qgg membrane ; the inner half fuses with the 

 second polar nucleus to form the syntelosome (Figs.c^,6,s). 

 The female pronucleus travels inwards and becomes 

 buried in the yolky material. The syntelosome contains 

 either 14 or 16 chromosomes (the author is not quite 

 certain) and remains as a group of isolated chromosomes 

 as far as the blastoderm stage. 



Nematus lacteus. (This species is probably arrheno- 

 tokous, as it closely resembles JV. pavidus^ which produces 

 males from unfertilised eggs.) The maturation and fate 

 of the polar bodies of the unfertilised &gg of this species 

 are very similar to that of A^. ribesii. There is not, how- 

 ever, a complete fusion of the inner half of the first polar 

 nucleus with the second polar nucleus. The two groups 

 of eight chromosomes lie side by side. One of these soon 

 disappears, the other persisting a little longer. In the 

 thelyotokous species Poecilosoma liiteolunt, Hemidiroa 

 rufa (which may produce a few males) and Croesus varus, 

 there is no conjugation of the inner half of the first polar 



